Eraser

Most people know (or should know) that if you delete a file, it's often recoverable with the proper tools. Even if you've written data over the exact spot on your drive where the file formerly existed, there's a good chance that an expert can recover it. Additionally, Windows squirrels temp files all over the place, and there's a good chance that multiple records exist in more than one place. Thus the need for secure file deletion.

Eraser is an excellent product and is probably overkill, but you can be confident it will get the job done as well as possible. It writes over the original file with random data several times, it purges the paging file, temp file, etc. It can also wipe a disk clean or clean out unused space. Short of chopping up your drive with tin snips, this is about as good as it can get.

http://www.heidi.ie/eraser/