Let's say you have a had drive or pc laying around somewhere. Whether you want to reuse it, sell it, give it to a friend, whatever, you NEED to make sure that data is cleared off of it. Alot of people think that by just deleting a file from windows and cleaning out the recycle bin gets rid of the file forever.
I've had a few scenarios where people brought me an old hard drive that "doesn't work" and I've been able to pull massive amounts of data off of it using nothing but a free piece of software I found on the interwebz. Lucky for them I work for the forces of good rather than evil....sometimes. Anyway, if you're in a situation where you need to completely clear a disk of all of it's contents, please check out the article below from Lifehacker.com . Give's you an idea of what happens when you delete a file, and how to securely wipe a drive of data. If you really want a want to make sure that no one gets data from your drive, I will post a blog soon with pictures on how to open your HDD and wreak havoc on it like Charlie Sheen on a pile of coke. Enjoy the Article :)
LifeHacker - How to Properly Erase Your Physical Media