Installing Service Pack 1 on Windows Vista archives certain files so you can remove SP1 later if you decide to. The likelihood of your wanting do this however is slim since SP1 significantly improves Vista’s performance and behavior. So if you’ve installed SP1 and have verified that all your devices and applications are working properly, you can remove these archived files by using the VSP1CLN command which is new to Vista SP1. For more information type VSP1CLN /? at a command prompt. Note that once you run this tool you won’t be able to uninstall SP1, so be sure to test your upgrade carefully before you run the command.
Want to know more about Vista internals and how to administer Vista in enterprise environments? Get the Windows Vista Resource Kit, Second Edition from Microsoft Press.