If you don’t want Windows XP Service Pack 3, and Vista Service Pack 1, to be installed consider installing the Windows Service Pack Blocker Tool Kit. It will temporarily prevent installation of Service Pack updates through Windows Update.
“…This tool can be used with:
* Windows Server 2003 Service Pack 2 (valid through March, 2008)
* Windows XP Service Pack 3 (valid for 12 months following general availability)
* Windows Vista Service Pack 1 (valid for 12 months following general availability)
“…this toolkit will not prevent the installation of the service pack from CD/DVD, or from the stand-alone download package. This simply prevents the service pack from being delivered over Windows Update.”