How to opt out of Windows Vista SP1
So Microsoft has issued a tool that will let users block Windows Update from automatically installing service packs. The tool isn't actually Windows Vista specific. It also works with Windows XP (to block SP3), and Windows Server 2003 (to block SP2).
There's no option to prevent your PC from downloading updates permanently. The Windows Server 2003 blocker will only work through March, while the Vista and XP tools will keep you from downloading the updates for up to 12 months following the official release of XP SP3 and Vista SP1.