1. Deleting a file only deletes the file's entry in the directory and not the content of the file. Read http://www.x-ways.net/winhex/forum/messages/6/12.html?1039253269 about WINHEX which from memory will write zeros or patterns to such space.
2. If it's in a file that you don't know about, then that's an issue...
3. Many donate machines with some OS installed, but fail to supply the OS and its license. This can be a confounding problem to the recipient. "Do I have a license to this OS? or not?"
Maybe you should just zero fill the drive, install some Linux on it so they can see it boots?
I'm considering donating my computer and I heard that some things can still be on your computer after you delete them. Does anyone know how I can permenantly delete things to protect any personal information?