<?xml version="1.0" encoding="us-ascii"?><InstanceReport xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"><Version>2.4.0.8</Version><ReportLongName>060600 - Disclosure - Deferred Service Contract Revenue</ReportLongName><DisplayLabelColumn>true</DisplayLabelColumn><ShowElementNames>false</ShowElementNames><RoundingOption /><HasEmbeddedReports>false</HasEmbeddedReports><Columns><Column FlagID="0"><Id>1</Id><IsAbstractGroupTitle>false</IsAbstractGroupTitle><LabelSeparator>

</LabelSeparator><CurrencyCode /><FootnoteIndexer /><hasSegments>false</hasSegments><hasScenarios>false</hasScenarios><MCU><KeyName /><CurrencySymbol /><contextRef><ContextID>c20130101to20130630</ContextID><EntitySchema>http://www.sec.gov/CIK</EntitySchema><EntityValue>0001162461</EntityValue><PeriodDisplayName /><PeriodType>duration</PeriodType><PeriodStartDate>2013-01-01T00:00:00</PeriodStartDate><PeriodEndDate>2013-06-30T00:00:00</PeriodEndDate><Segments /><Scenarios /></contextRef><UPS /><CurrencyCode /><OriginalCurrencyCode /></MCU><CurrencySymbol /><Labels><Label Key="CalendarSupplement" Id="0" Label="6 Months Ended" /><Label Key="Calendar" Id="1" Label="Jun. 30, 2013" /></Labels></Column></Columns><Rows><Row FlagID="0"><Id>1</Id><IsAbstractGroupTitle>true</IsAbstractGroupTitle><LabelSeparator>

</LabelSeparator><Level>1</Level><ElementName>us-gaap_DeferredRevenueDisclosureAbstract</ElementName><ElementPrefix>us-gaap_</ElementPrefix><IsBaseElement>true</IsBaseElement><BalanceType>na</BalanceType><PeriodType>duration</PeriodType><IsReportTitle>false</IsReportTitle><IsSegmentTitle>false</IsSegmentTitle><IsCalendarTitle>false</IsCalendarTitle><IsEquityPrevioslyReportedAsRow>false</IsEquityPrevioslyReportedAsRow><IsEquityAdjustmentRow>false</IsEquityAdjustmentRow><IsBeginningBalance>false</IsBeginningBalance><IsEndingBalance>false</IsEndingBalance><IsReverseSign>false</IsReverseSign><FootnoteIndexer /><Cells><Cell FlagID="0" ContextID="" UnitID=""><Id>1</Id><IsNumeric>false</IsNumeric><IsRatio>false</IsRatio><DisplayZeroAsNone>false</DisplayZeroAsNone><NumericAmount>0</NumericAmount><RoundedNumericAmount>0</RoundedNumericAmount><NonNumbericText /><FootnoteIndexer /><CurrencyCode /><CurrencySymbol /><IsIndependantCurrency>false</IsIndependantCurrency><ShowCurrencySymbol>false</ShowCurrencySymbol><DisplayDateInUSFormat>false</DisplayDateInUSFormat></Cell></Cells><ElementDataType>xbrli:stringItemType</ElementDataType><SimpleDataType>string</SimpleDataType><IsTotalLabel>false</IsTotalLabel><UnitID>0</UnitID><Label>Deferred Service Contract Revenue [Abstract]</Label></Row><Row FlagID="0"><Id>2</Id><IsAbstractGroupTitle>false</IsAbstractGroupTitle><LabelSeparator>

</LabelSeparator><Level>2</Level><ElementName>us-gaap_DeferredRevenueDisclosureTextBlock</ElementName><ElementPrefix>us-gaap_</ElementPrefix><IsBaseElement>true</IsBaseElement><BalanceType>na</BalanceType><PeriodType>duration</PeriodType><IsReportTitle>false</IsReportTitle><IsSegmentTitle>false</IsSegmentTitle><IsCalendarTitle>false</IsCalendarTitle><IsEquityPrevioslyReportedAsRow>false</IsEquityPrevioslyReportedAsRow><IsEquityAdjustmentRow>false</IsEquityAdjustmentRow><IsBeginningBalance>false</IsBeginningBalance><IsEndingBalance>false</IsEndingBalance><IsReverseSign>false</IsReverseSign><PreferredLabelRole>verboseLabel</PreferredLabelRole><FootnoteIndexer /><Cells><Cell FlagID="0" ContextID="c20130101to20130630" UnitID=""><Id>1</Id><IsNumeric>false</IsNumeric><IsRatio>false</IsRatio><DisplayZeroAsNone>false</DisplayZeroAsNone><NumericAmount>0</NumericAmount><RoundedNumericAmount>0</RoundedNumericAmount><NonNumbericText>&lt;div style="font-family: 'Times New Roman', Times, serif; font-size: 10pt;"&gt;&lt;div style="text-align: left; font-family: ''Times New Roman'', Times, serif; font-size: 10pt; font-weight: bold;"&gt;Note 6. Deferred Service Contract Revenue&lt;/div&gt;&lt;div&gt;&lt;br /&gt;&lt;/div&gt;&lt;div style="text-align: left; font-family: ''Times New Roman'', Times, serif; font-size: 10pt;"&gt;Service contract revenue is recognized on a straight-line basis over the period of the applicable extended warranty contract.&lt;/div&gt;&lt;div&gt;&lt;br /&gt;&lt;/div&gt;&lt;div style="text-align: left; font-family: ''Times New Roman'', Times, serif; font-size: 10pt;"&gt;The following table provides changes in deferred service contract revenue for the six-month periods ended June 30, 2013 and 2012 &lt;font style="font-style: italic; font-family: ''Times New Roman'', Times, serif; font-size: 10pt;"&gt;(in thousands)&lt;/font&gt;:&lt;/div&gt;&lt;div&gt;&lt;br /&gt;&lt;/div&gt;&lt;table cellpadding="0" cellspacing="0" style="width: 100%; font-family: 'Times New Roman', Times, serif; font-size: 10pt;"&gt;&lt;tr&gt;&lt;td valign="bottom" style="padding-bottom: 2px; vertical-align: bottom;"&gt;&lt;div style="margin-left: 9pt;"&gt;&amp;#160;&lt;/div&gt;&lt;/td&gt;&lt;td valign="bottom" style="padding-bottom: 2px; vertical-align: bottom;"&gt;&amp;#160;&lt;/td&gt;&lt;td colspan="6" valign="bottom" style="border-bottom: #000000 2px solid; vertical-align: bottom;"&gt;&lt;div style="text-align: center; font-family: ''Times New Roman'', Times, serif; font-size: 10pt; font-weight: bold;"&gt;June 30&lt;/div&gt;&lt;/td&gt;&lt;td nowrap="nowrap" valign="bottom" style="text-align: left; padding-bottom: 2px; vertical-align: bottom;"&gt;&amp;#160;&lt;/td&gt;&lt;/tr&gt;&lt;tr&gt;&lt;td valign="bottom" style="padding-bottom: 2px; vertical-align: bottom;"&gt;&lt;div&gt;&amp;#160;&lt;/div&gt;&lt;/td&gt;&lt;td valign="bottom" style="padding-bottom: 2px; vertical-align: bottom;"&gt;&amp;#160;&lt;/td&gt;&lt;td colspan="2" valign="bottom" style="border-bottom: #000000 2px solid; vertical-align: bottom;"&gt;&lt;div style="text-align: center; font-family: ''Times New Roman'', Times, serif; font-size: 10pt; font-weight: bold;"&gt;2013&lt;/div&gt;&lt;/td&gt;&lt;td nowrap="nowrap" valign="bottom" style="text-align: left; padding-bottom: 2px; vertical-align: bottom;"&gt;&amp;#160;&lt;/td&gt;&lt;td valign="bottom" style="padding-bottom: 2px; vertical-align: bottom;"&gt;&amp;#160;&lt;/td&gt;&lt;td colspan="2" valign="bottom" style="border-bottom: #000000 2px solid; vertical-align: bottom;"&gt;&lt;div style="text-align: center; font-family: ''Times New Roman'', Times, serif; font-size: 10pt; font-weight: bold;"&gt;2012&lt;/div&gt;&lt;/td&gt;&lt;td nowrap="nowrap" valign="bottom" style="text-align: left; padding-bottom: 2px; vertical-align: bottom;"&gt;&amp;#160;&lt;/td&gt;&lt;/tr&gt;&lt;tr&gt;&lt;td valign="bottom" style="background-color: #cceeff; width: 76%; vertical-align: bottom;"&gt;&lt;div style="text-align: left; text-indent: -16.2pt; font-family: ''Times New Roman'', Times, serif; margin-left: 16.2pt; font-size: 10pt;"&gt;Beginning Balance&lt;/div&gt;&lt;/td&gt;&lt;td valign="bottom" style="background-color: #cceeff; width: 1%; vertical-align: bottom;"&gt;&amp;#160;&lt;/td&gt;&lt;td valign="bottom" style="text-align: left; background-color: #cceeff; width: 1%; vertical-align: bottom;"&gt;&lt;div style="font-family: ''Times New Roman'', Times, serif; font-size: 10pt;"&gt;$&lt;/div&gt;&lt;/td&gt;&lt;td valign="bottom" style="text-align: right; background-color: #cceeff; width: 9%; vertical-align: bottom;"&gt;&lt;div style="font-family: ''Times New Roman'', Times, serif; font-size: 10pt;"&gt;8,539&lt;/div&gt;&lt;/td&gt;&lt;td nowrap="nowrap" valign="bottom" style="text-align: left; background-color: #cceeff; width: 1%; vertical-align: bottom;"&gt;&amp;#160;&lt;/td&gt;&lt;td valign="bottom" style="background-color: #cceeff; width: 1%; vertical-align: bottom;"&gt;&amp;#160;&lt;/td&gt;&lt;td valign="bottom" style="text-align: left; background-color: #cceeff; width: 1%; vertical-align: bottom;"&gt;&lt;div style="font-family: ''Times New Roman'', Times, serif; font-size: 10pt;"&gt;$&lt;/div&gt;&lt;/td&gt;&lt;td valign="bottom" style="text-align: right; background-color: #cceeff; width: 9%; vertical-align: bottom;"&gt;&lt;div style="font-family: ''Times New Roman'', Times, serif; font-size: 10pt;"&gt;5,838&lt;/div&gt;&lt;/td&gt;&lt;td nowrap="nowrap" valign="bottom" style="text-align: left; background-color: #cceeff; width: 1%; vertical-align: bottom;"&gt;&amp;#160;&lt;/td&gt;&lt;/tr&gt;&lt;tr&gt;&lt;td valign="bottom" style="background-color: #ffffff; width: 76%; vertical-align: bottom;"&gt;&lt;div style="text-align: left; text-indent: -16.2pt; font-family: ''Times New Roman'', Times, serif; margin-left: 25.2pt; font-size: 10pt;"&gt;Add: &amp;#160; &amp;#160; &amp;#160; &amp;#160; Payments received&lt;/div&gt;&lt;/td&gt;&lt;td valign="bottom" style="background-color: #ffffff; width: 1%; vertical-align: bottom;"&gt;&amp;#160;&lt;/td&gt;&lt;td valign="bottom" style="text-align: left; background-color: #ffffff; width: 1%; vertical-align: bottom;"&gt;&amp;#160;&lt;/td&gt;&lt;td valign="bottom" style="text-align: right; background-color: #ffffff; width: 9%; vertical-align: bottom;"&gt;&lt;div style="font-family: ''Times New Roman'', Times, serif; font-size: 10pt;"&gt;7,140&lt;/div&gt;&lt;/td&gt;&lt;td nowrap="nowrap" valign="bottom" style="text-align: left; background-color: #ffffff; width: 1%; vertical-align: bottom;"&gt;&amp;#160;&lt;/td&gt;&lt;td valign="bottom" style="background-color: #ffffff; width: 1%; vertical-align: bottom;"&gt;&amp;#160;&lt;/td&gt;&lt;td valign="bottom" style="text-align: left; background-color: #ffffff; width: 1%; vertical-align: bottom;"&gt;&amp;#160;&lt;/td&gt;&lt;td valign="bottom" style="text-align: right; background-color: #ffffff; width: 9%; vertical-align: bottom;"&gt;&lt;div style="font-family: ''Times New Roman'', Times, serif; font-size: 10pt;"&gt;6,033&lt;/div&gt;&lt;/td&gt;&lt;td nowrap="nowrap" valign="bottom" style="text-align: left; background-color: #ffffff; width: 1%; vertical-align: bottom;"&gt;&amp;#160;&lt;/td&gt;&lt;/tr&gt;&lt;tr&gt;&lt;td valign="bottom" style="background-color: #cceeff; width: 76%; vertical-align: bottom;"&gt;&lt;div style="text-align: left; text-indent: -16.2pt; font-family: ''Times New Roman'', Times, serif; margin-left: 25.2pt; font-size: 10pt;"&gt;Add: &amp;#160; &amp;#160; &amp;#160; &amp;#160; Contract revenue assumed with business acquisition&lt;/div&gt;&lt;/td&gt;&lt;td valign="bottom" style="background-color: #cceeff; width: 1%; vertical-align: bottom;"&gt;&amp;#160;&lt;/td&gt;&lt;td valign="bottom" style="text-align: left; background-color: #cceeff; width: 1%; vertical-align: bottom;"&gt;&amp;#160;&lt;/td&gt;&lt;td valign="bottom" style="text-align: right; background-color: #cceeff; width: 9%; vertical-align: bottom;"&gt;&lt;div style="font-family: ''Times New Roman'', Times, serif; font-size: 10pt;"&gt;&amp;#8212;&lt;/div&gt;&lt;/td&gt;&lt;td nowrap="nowrap" valign="bottom" style="text-align: left; background-color: #cceeff; width: 1%; vertical-align: bottom;"&gt;&amp;#160;&lt;/td&gt;&lt;td valign="bottom" style="background-color: #cceeff; width: 1%; vertical-align: top;"&gt;&amp;#160;&lt;/td&gt;&lt;td valign="bottom" style="text-align: left; background-color: #cceeff; width: 1%; vertical-align: top;"&gt;&amp;#160;&lt;/td&gt;&lt;td valign="bottom" style="text-align: right; background-color: #cceeff; width: 9%; vertical-align: top;"&gt;&lt;div style="font-family: ''Times New Roman'', Times, serif; font-size: 10pt;"&gt;780&lt;/div&gt;&lt;/td&gt;&lt;td nowrap="nowrap" valign="bottom" style="text-align: left; background-color: #cceeff; width: 1%; vertical-align: top;"&gt;&amp;#160;&lt;/td&gt;&lt;/tr&gt;&lt;tr&gt;&lt;td valign="bottom" style="padding-bottom: 2px; background-color: #ffffff; width: 76%; vertical-align: bottom;"&gt;&lt;div style="text-align: left; text-indent: -16.2pt; font-family: ''Times New Roman'', Times, serif; margin-left: 25.2pt; font-size: 10pt;"&gt;Less: &amp;#160; &amp;#160; &amp;#160; &amp;#160; Revenue recognized&lt;/div&gt;&lt;/td&gt;&lt;td valign="bottom" style="padding-bottom: 2px; background-color: #ffffff; width: 1%; vertical-align: bottom;"&gt;&amp;#160;&lt;/td&gt;&lt;td valign="bottom" style="border-bottom: #000000 2px solid; text-align: left; background-color: #ffffff; width: 1%; vertical-align: bottom;"&gt;&amp;#160;&lt;/td&gt;&lt;td valign="bottom" style="border-bottom: #000000 2px solid; text-align: right; background-color: #ffffff; width: 9%; vertical-align: bottom;"&gt;&lt;div style="font-family: ''Times New Roman'', Times, serif; font-size: 10pt;"&gt;(5,964&lt;/div&gt;&lt;/td&gt;&lt;td nowrap="nowrap" valign="bottom" style="text-align: left; padding-bottom: 2px; background-color: #ffffff; width: 1%; vertical-align: bottom;"&gt;&lt;div style="font-family: ''Times New Roman'', Times, serif; font-size: 10pt;"&gt;)&lt;/div&gt;&lt;/td&gt;&lt;td valign="bottom" style="padding-bottom: 2px; background-color: #ffffff; width: 1%; vertical-align: bottom;"&gt;&amp;#160;&lt;/td&gt;&lt;td valign="bottom" style="border-bottom: #000000 2px solid; text-align: left; background-color: #ffffff; width: 1%; vertical-align: bottom;"&gt;&amp;#160;&lt;/td&gt;&lt;td valign="bottom" style="border-bottom: #000000 2px solid; text-align: right; background-color: #ffffff; width: 9%; vertical-align: bottom;"&gt;&lt;div style="font-family: ''Times New Roman'', Times, serif; font-size: 10pt;"&gt;(5,645&lt;/div&gt;&lt;/td&gt;&lt;td nowrap="nowrap" valign="bottom" style="text-align: left; padding-bottom: 2px; background-color: #ffffff; width: 1%; vertical-align: bottom;"&gt;&lt;div style="font-family: ''Times New Roman'', Times, serif; font-size: 10pt;"&gt;)&lt;/div&gt;&lt;/td&gt;&lt;/tr&gt;&lt;tr&gt;&lt;td valign="bottom" style="padding-bottom: 4px; background-color: #cceeff; width: 76%; vertical-align: bottom;"&gt;&lt;div style="text-align: left; text-indent: -16.2pt; font-family: ''Times New Roman'', Times, serif; margin-left: 16.2pt; font-size: 10pt;"&gt;Ending Balance&lt;/div&gt;&lt;/td&gt;&lt;td valign="bottom" style="padding-bottom: 4px; background-color: #cceeff; width: 1%; vertical-align: bottom;"&gt;&amp;#160;&lt;/td&gt;&lt;td valign="bottom" style="border-bottom: #000000 4px double; text-align: left; background-color: #cceeff; width: 1%; vertical-align: bottom;"&gt;&lt;div style="font-family: ''Times New Roman'', Times, serif; font-size: 10pt;"&gt;$&lt;/div&gt;&lt;/td&gt;&lt;td valign="bottom" style="border-bottom: #000000 4px double; text-align: right; background-color: #cceeff; width: 9%; vertical-align: bottom;"&gt;&lt;div style="font-family: ''Times New Roman'', Times, serif; font-size: 10pt;"&gt;9,715&lt;/div&gt;&lt;/td&gt;&lt;td nowrap="nowrap" valign="bottom" style="text-align: left; padding-bottom: 4px; background-color: #cceeff; width: 1%; vertical-align: bottom;"&gt;&amp;#160;&lt;/td&gt;&lt;td valign="bottom" style="padding-bottom: 4px; background-color: #cceeff; width: 1%; vertical-align: bottom;"&gt;&amp;#160;&lt;/td&gt;&lt;td valign="bottom" style="border-bottom: #000000 4px double; text-align: left; background-color: #cceeff; width: 1%; vertical-align: bottom;"&gt;&lt;div style="font-family: ''Times New Roman'', Times, serif; font-size: 10pt;"&gt;$&lt;/div&gt;&lt;/td&gt;&lt;td valign="bottom" style="border-bottom: #000000 4px double; text-align: right; background-color: #cceeff; width: 9%; vertical-align: bottom;"&gt;&lt;div style="font-family: ''Times New Roman'', Times, serif; font-size: 10pt;"&gt;7,006&lt;/div&gt;&lt;/td&gt;&lt;td nowrap="nowrap" valign="bottom" style="text-align: left; padding-bottom: 4px; background-color: #cceeff; width: 1%; vertical-align: bottom;"&gt;&amp;#160;&lt;/td&gt;&lt;/tr&gt;&lt;/table&gt;&lt;div&gt;&lt;br /&gt;&lt;/div&gt;&lt;div style="text-align: left; font-family: ''Times New Roman'', Times, serif; font-size: 10pt;"&gt;Costs incurred under service contracts were $3.5 million and $3.6 million for the six-month periods ended June 30, 2013 and 2012, which were recognized as incurred.&lt;/div&gt;&lt;/div&gt;</NonNumbericText><FootnoteIndexer /><CurrencyCode /><CurrencySymbol /><IsIndependantCurrency>false</IsIndependantCurrency><ShowCurrencySymbol>false</ShowCurrencySymbol><DisplayDateInUSFormat>false</DisplayDateInUSFormat></Cell></Cells><ElementDataType>nonnum:textBlockItemType</ElementDataType><SimpleDataType>na</SimpleDataType><ElementDefenition>The entire disclosure for deferred revenues at the end of the reporting period, and description and amounts of significant changes that occurred during the reporting period. Deferred revenue is a liability as of the balance sheet date related to a revenue producing activity for which revenue has not yet been recognized. Generally, an entity records deferred revenue when it receives consideration from a customer before achieving certain criteria that must be met for revenue to be recognized in conformity with GAAP.</ElementDefenition><ElementReferences>Reference 1: http://www.xbrl.org/2003/role/presentationRef

 -Publisher FASB

 -Name Accounting Standards Codification

 -Topic 210

 -SubTopic 10

 -Section 45

 -Paragraph 8

 -Subparagraph (b)

 -URI http://asc.fasb.org/extlink&amp;oid=6361293&amp;loc=d3e6935-107765



Reference 2: http://www.xbrl.org/2003/role/presentationRef

 -Publisher SEC

 -Name Staff Accounting Bulletin (SAB)

 -Number Topic 13

 -Section A



Reference 3: http://www.xbrl.org/2003/role/presentationRef

 -Publisher FASB

 -Name Accounting Standards Codification

 -Topic 605

 -SubTopic 10

 -Section S99

 -Paragraph 1

 -Subparagraph (SAB TOPIC 13.A.3(b).Q1(c),(b).Q2,(c).Q3)

 -URI http://asc.fasb.org/extlink&amp;oid=6600647&amp;loc=d3e214044-122780



Reference 4: http://www.xbrl.org/2003/role/presentationRef

 -Publisher FASB

 -Name Statement of Financial Accounting Concepts (CON)

 -Number 5

 -Paragraph 83, 84

 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.  This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.



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