There should be a hardware test DVD that came with it, insert that into the optical drive, and boot off the DVD. Once you have done that and are running off the DVD, go into Disk Utility and select your drive. Then select the option 'Zero All Data'. It should take a while, but once it's done it will be impossible to recover data with programs like Norton.
(the data can still be recovered, but it takes thousands and thousands of dollars to do it, so I wouldn't worry about it)