I'm trying to defrag a machine, but I'm having trouble. It looks to me like a whole lot of deleted stuff is hanging around on the drive, taking up space that would be nice to have for moving things around. The drive is a little over 5GB, and everything is showing that about a quarter of it is empty, but the graphical portion of the defrag is showing that there's a very tiny amount of actual free space to move things around with.

Is my assumption correct? Is there stuff hanging on? Is there a way to remove it?