Download SP3 to a folder such as XP-SP3, then execute the following:
C:\XP-SP3\WindowsXP-KB936929-SP3-x86-ENU.exe -x
XP SP3 download:http://www.microsoft.com/downloads/details.aspx?FamilyId=5B33B5A8-5E76-401F-BE08-1E1555D4F3D4&displaylang=en
XP SP3 Overview: http://www.microsoft.com/downloads/details.aspx?FamilyId=68C48DAD-BC34-40BE-8D85-6BB4F56F5110&displaylang=en