Run this a few times
http://www.dban.org/
Then destroy the platters
http://www.popularmechanics.com/tech...o/4212242.html
If your not going to go the extra mile to be paranoid why bother?
But you could just be a slacker and whack the shit out of it with a hammer that will deter your casual dumpster diver from retrieving any info that remains. ( And there is still info there trust me a regular format leaves alot behind )