This is a complex issue. The best thing to do is re-install the OS using a clean install option, install ShredIt, and then shred the free space. That way you remove all the complexity from the problem. Another option would be to add a new admin user, login to the new user, delete all the other users, and then shred the free space. This is how I get a machine ready for eBay. I even name the new user eBay.