What operating system is it? If it's Vista, then there should be something in the Control Panel, called Full Destructive Recovery. It basically resets the computer back to the factory settings, so all that's left is the basic system as it would be if the computer was bought new. I narrowly avoided doing that myself (and losing all my files) recently after problems whenever I started my laptop up.
But yeah, Full Destructive Recovery. I dare say it's something similar for other Windows operating systems too, though I can't be sure since I don't use any others. If you have a non-Windows OS though, then I can't really help you here, sorry.