Use eraser, its a small, free, very secure cleaning software. It rewrites garbage data on the entire drive. Two or three passes is enough to securely delete the data, beyond recovery. It is still theoretically possible by some kind of forensics team, but no one would invest that kind of money. FreeCommander is another alternative. It would, however, have been faster to erase the written data using any of these software, that way, you would not have to clean the entire disk.