Defragging the drive might help a bit (but takes ages!), but as Sam said, it sounds more like the Operating System taking up the space. Is it a mass-market shop bought PC? They usually have some of the OS stored in a hidden partition, so that they only give you a recovery CD, instead of the full Windows installation CD. That could also account for some of the space, although that would usually only be on your main (C) drive. Do you also have an Operating System installed on your E drive?