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 XP Service Pack 3 (valid until May 19th, 2009)
Windows Vista Service Pack 1 (valid until April 28th, 2009)
Windows Vista Service Pack 2 (valid for 12 months following general availability)
Windows Server 2008 Service Pack 2 (valid for 12 months following general availability)
Microsoft Download