Cool tool: Boot and Nuke
I use BCWipe for deleting individual files and directories, but you should always wipe hard disks completely before giving away or selling a used computer.
The Darik's Boot and Nuke free boot floppy and CD does just that:
Darik's Boot and Nuke ("DBAN") is a self-contained boot floppy that securely wipes the hard disks of most computers. DBAN will automatically and completely delete the contents of any hard disk that it can detect, which makes it an appropriate utility for bulk or emergency data destruction.
As the instructions says: Please clearly label your DBAN boot media because it is dangerous.