There's been a bunch of articles in the press recently about privacy concerns, data being left on old computers, etc, etc. Most of these have mentioned expensive 'cleaning' tools for wiping hard drive contents, and they often mention the need to overwrite empty segments a number of times to garuantee their deletion.
There is a DOS command that will do this for you - I *think* it's post SP1 for XP. It's called cipher, and it overwrites blank segments three times - first with 0, then with 1, then with random numbers. Use the "/w" option.