How can I make sure everything is deleted off my hard drive before I sell my computer?
Answer: The quickest way to "erase" a hard disk is to initialize it. This process clears the current file system and creates a new directory structure. However, the files on disk are not actually removed -- only their references are gone. This means it is possible to retrieve many of the files on a newly initialized drive.
In order to make sure all your data is truly deleted, you need to "wipe" your hard disk. If you use a Macintosh, the Disk Utility program allows you to choose "Zero All Data" or perform an "8 Way Random Write" when erasing a disk. Either of these options will effectively make your data unrecoverable. Windows does not have a built-in option for wiping the hard drive, but there are several utility programs that will do the job for you.
An example is Darik's Boot and Nuke. Several others can be found at CNET's Download.com. If you want to keep any of your data before wiping the drive, be sure to back it up on another drive since you will not be able to recover it.