as described below you can use vsp1cln.exe to remove SP1 uninstallation information and recover up to 1Gb of disk space...

Windows Vista SP1 File Removal tool (Vsp1cln.exe). Vsp1cln.exe is an optional tool that you can run
after you install SP1. This tool removes older versions of components that have been updated in SP1,
which are stored during the installation in case you need to uninstall SP1 later. Saving these older
components increases the amount of disk space that is used. Typically, you should run Vsp1cln.exe if
you want to reclaim this disk space after applying SP1 and if you will not need to uninstall SP1. Note,
however, that you cannot uninstall SP1 after you run this tool. You can use this tool both online and
offline, but you must have SP1 installed and you must use the correct version of the tool. If you are
running this file on an offline image, you should use the Vsp1cln.exe included in the Windows OPK or
AIK. However, if you are running this file online (on a computer that you are booted into), you should
use the Vsp1cln.exe file located at %windir%\system32\vsp1cln.exe.

More info: http://technet2.microsoft.com/WindowsVista/en/library/5cf710a1-1414-4d11-97de-
0748abdcec651033.mspx?mfr=true