If you have installed Windows Vista SP1 on top of your first Vista installation and have no plans to uninstall it, you might want to give the following command a try “VSP1CLN.exe”.
The Windows Vista Service Pack 1 (SP1) Files Removal Tool (VSP1CLN.exe) can be used to remove the files that are archived after Windows Vista SP1 is applied. Running this tool is optional.
Installing Windows Vista SP1 increases the amount of disk space that is used by the operating system. This space is used to archive files so that SP1 can be uninstalled. 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.
More details on Microsoft TechNet: http://technet.microsoft.com/en-us/library/cc709655.aspx