Engadget: How-to Erase Old Hard Disks
Every computer that I have ever been given has always had things on the hard drive. Usually this incluced the entire operation system plus any and all user files. I usually take these old computers and immediately install Linux onto them removing all the old data. However, this does not mean that the old information is beyond retrieval.
Here is a tutorial outlining a quick and dirty way to affectively erase an old hard drive.