If you are running Windows Vista and have installed Service Pack 1 (SP1), you can gain around 800mb of disk space back.
When you install SP1, it backs up all the files in case you want to remove SP1. But chances are pretty good that once you have SP1 installed you really have no reason to ever install it, so it is safe to remove those backup files.
It’s pretty simple. Press Winkey+R to pull up a run dialog. Type in Vsp1cln and press enter.
A prompt will confirm deleted the files. Enter y obviously and it goes to work removing these useless backup files.
Enjoy reclaiming a bit of your hard drive space!