You should have a recovery disk that came with your computer, or should have been prompted to make one. You'll need to find that before you continue.
You back up all your documents that you want to keep - everything on the computer at the moment will be deleted. If you've got an external hard drive you can copy all the files you need across onto that, or another computer - anything that'll hold them really. Usually this involves the contents of your "My documents" folder, and if you keep any documents anywhere else on your computer back those up too.
Then, just put the CD in the drive and reboot. It should boot from the CD and walk you through formatting the hard drive and then reinstalling windows.