Microsoft released Windows Service Pack Blocker Tool Kit
Windows Service Pack Blocker Tool Kit - A blocking tool is available for organizations that would like to 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)
You can download it from http://www.microsoft.com/downloads/details.aspx?FamilyID=d7c9a07a-5267-4bd6-87d0-e2a72099edb7&DisplayLang=en