When a file is "deleted" from your hard drive, it's not really erased. All that's really happening is the reference or pointer to that file is removed from the hard drive's table-of-contents. (this is an oversimplification, but essentially true)
So, if you can browse around the hard drive rather than use the TOC to find things (the normal OS and Explorer and such use the TOC) you can often find interesting things.
There are other more advanced data-recovery techniques, up to and including use of a scanning electron microscope to physically look at the surface of the disk and determine the orientation of the magnetic particles which denote 0 and 1. Obviously the more exotic techniques cost big money to use.
So, i suggest you cruise around Google looking for things like "deleted hard drive recovery" or "file restore" and such..