Answer for now many times can the software be used on hard drives and can hard drives be reformatted is there help with learning which OS files that can/need to be delete from hard drive

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.