Microsoft offer's the Windows Service Pack Blocker Tool Kit
This tool can be used to block the following service packs from automatic installation for approx 12 months (see listed expiriations)
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)
http://www.microsoft.com/downloads/details.aspx?familyid=d7c9a07a-5267-4bd6-87d0-e2a72099edb7&displaylang=en&tm