1. there is no way to actually delete information from a hard drive, all you can do is write over it. when you delete a file, windows marks the space as "open" and eventually writes over it, however the information that was formerly occupying that space can still be recovered via special software.
2. when you use a "wipe" or "zerowrite" program, it writes zeros over the entire drive. every time you do this it gets more and more difficult to recover the data that was previously there. after several "zerowrites", it should become impossible to recover the old data using commercially available software.
3. the only true way to make sure all traces of data are gone is physically destroy the drive (however that might make it a bit more difficult to sell

)
4. after doing [2.] or [3.], the only ones who can recover any of your data at that point is the DoD, and unless you're doing some real shady things then you should have nothing to worry about
cheers :wave: