Here is a set of DOS-based file-wiping utilities that you can use to clean excess junk out of the "unused" space of your hard disk. This package is very small – it's only 9K in size! – but it's very powerful. It's a series of simple programs that will wipe out traces of deleted files that still exist in the unused "free" space on your hard disk, and overwrite it a number of times (you pick the number of times) to ensure that it is completely gone.
To install these programs, simply create a subfolder on your hard drive and unzip this file into there. The programs will only run under DOS, so Windows users (including XP) will need to start a command window in order to run them.