There is software available to wipe your hard drive. What you really want to do is traverse three or four rounds of putting random 1's and 0's on every sector of your hard drive. That way, no one will be able to recover anything you had on there, like personal correspondence, passwords, credit card numbers, etc.

There is some software called Paragon Disk Wiper that works on the most recent versions of Windows, except Vista, but I'm betting it would work there too. You can download a free copy at:
http://www.newfreedownloads.com/find/disk-wipe.html.

I highly recommend some sort of disk wipe software because a format of the hard drive (unless you do a low-level format) won't really get rid of everything.

Regards,
MS