Are you selling your old system, donating a computer or discarding an old business system? Deleting the data on the hard drive doesn't prevent a recovery program from restoring it. Wipe the data from the hard disk the right way.

DataEraser
http://www.ontrack.com/dataeraser/

Active KillDisk
http://www.killdisk.com/

Disk Shredder
http://www.disksh.com/en/