This program is like no other I've ever used before - it's basicly a clean-up program, deleting files which are not at all needed and that take up space on your hard disk which could be used for more important things. These useless items also can noticeably slow down your computer if there are too many.
The first time I ran it, 500mb of useless files were deleted, and I noticed a significant improvement in the speed of my computer.
It also has a registry scanner, which deep scans the Windows registry and searches for invalid terms, and many more (normally left behind after installing/uninstalling programs). Again, these can really slow down your computer, and the first time I ran the registry scanner there were too many entries to count.
The program is CCleaner (Crap Cleaner). I've been using it for around 1 year now, and it has to be one, if not the best program I've used - clean interface, easy to use, very fast, and it has good results.
www.CCleaner.com
Hopefully you'll like it and I hope it's as good for you as it is for me :]

Mark