"The installation process of SP1 for Vista backs up all the files that it replaces (in order) to allow you to uninstall it (if & when needed).

If you are in dire need of free disk space you can use an integrated tool called Vsp1clean.exe (aka SP1 File Removal Tool)".

Note: If you are enthusiast with one of the time-limit beta/RC release version of Vista SP1, you need to be aware that removing the backed up files from above could result in the inability to uninstall the test release in lieu of the final SP1 version.