When you delete files or format your hard drive, did you know that the data is only marked for deletion but not actually deleted? There are several free data recovery programs such as Piriform’s Recuva that allows just about anyone to recover files deleted from your computer.
If you’re getting a new computer, make sure you’re careful about what happens with your old hard drive. If you have personal information on your computer, and most everyone does, simply deleting your documents and donating your old PC to charity or giving it to a friend is not a good idea.
You can, of course, physically destroy the hard drive with a sledge hammer. If that seems like a waste of a good hard drive, you can use a wiping program such as Darik’s Boot And Nuke (DBAN). With DBAN, you can do a Quick Erase which writes zeroes to every sector. A Quick Erase is usually adequate for internal redeployment. DBAN has more elaborate wiping algorithms such as the DoD Short which will do multiple passes and may take over a day to run.
If you want to be absolutely sure that your data is destroyed, you can use a degaussing machine then destroy it.
Or, you can use a hard drive shredder