You want this: http://www.dban.org/

By the way, this isn't really an appropriate question for HN.

Thanks for the suggestion.

However, I find the other part of your comment non-constructive. If you don't think that my question is inappropriate for HN, fine. But just writing it off without giving any justification or advice doesn't do anything to improve the community.

I normally use a Linux boot disk to boot in live CD and then I format my disk and use an utility that writes random bits on the drive. I can't remember how this utility is called but I think a quick google search will give a lot of results.

I've always been partial to DBAN:

http://www.dban.org/