As a Dad I'm pretty concerned about having my family's private information stolen. I'm also about to sell the family PC, which has loads of personal files on, like Excel/Word docs, saved bookmarks to my online accounts, etc. I've deleted all these, but a friend told me they aren't really gone and could be recovered with the use of some software.

What's the best way to ensure complete destruction of my hard drive's data, without chucking it on the fire?