After installing Vista SP1, recover hundreds of MBs of disk space with Vsp1cln.exe. You will lose ability to go back to RTM.

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 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. You can use this tool both online and offline, but you must have SP1 installed. You cannot uninstall SP1 after you run this tool on an image.

Originally spotted in here.

Please note that Vsp1cln.exe is included with Vista SP1 so there is nothing that needs to be downloaded.