When we install SP1, the installer keeps copies of the older versions of files it is replacing. This is done just in case the user wants to roll-back to the original Vista, without SP1. However if you are satisfied with the machine after installation of SP1 (like its running for a couple of weeks without any glitches), then its safe to assume that you will never want to uninstall SP1. So you can delete these unnecessary set of files and get back some free space.
Thankfully this feature is already provided with SP1 installer, so no need to delve deep into the file system or download third party tools. Just open the command prompt and run vsp1cln.exe. It takes little time to execute.