http://www.dban.org/
Dban is good for making sure everything is irrecoverable on the hard drive so that people can't access your files with recovery software. If you're selling the drive you wouldn't want somebody to be able to see your personal info. Also people might do it in order to destroy evidence if they have been downloading illegal stuff.
For your case you just need to format and reinstall windows. All the viruses will be gone and you can start out fresh. As you reinstall windows you just need to follow the directions on the screen to format your hard drive. A quick format should do the trick.