There are plenty of ways to do this safely. Im kind of in a hurry, so I wont link, but google for 'secure delete' or 'disk wipe'. The difference between deleting and a 'secure delete' is that when you just regularly delete something, all that happens is that the disk space is marked as usuable, so something else will eventually write over it. A 'secure delete' writes a bunch of random garbage over the disk space that was being used.

/edit damn, 2 beat me to the punch