If your plan is for the computer to leave your possession (even to another family member), a relatively safe approach is to purchase a replacement hard drive, swap the new hard drive for the current hard drive, and install a fresh OS from your install disks (not your back ups since they will include your personal information). Drill through the old disk drive in a number of places with a power drill, break external contacts with a pliers or hammer, and pour epoxy into the holes and over the broken contacts. You can also drop the old drive into a bucket of water after the epoxy hardens as a final step.