If you upgraded to Vista Service Pack 1 when it went live earlier this week, Windows left behind several files system files from your pre-SP1 install that aren't being used any longer. The files are left taking up space on your drive so that you can roll back SP1 if you wanted to, but since you're unlikely to grow sick of all those improvements, weblog Digital Inspiration details how to remove the unneeded junk files and free up space on your hard drive.
There are no manual steps involved. Just open your Windows command prompt and type vsp1cln.exe (short for Vista SP1 Cleaner). It takes less than a minute to execute.
Just remember, once you execute the SP1 cleaner, you won't be able to roll back, so make sure you're happy with SP1 before you start freeing up space.