Download this utility, it will show you exactly what is taking up space and where. After it is done scanning the larger blocks are bigger files, see what you can uncover using it first.
It's a decent enough utility, however, it does not show absolutely everything on a drive. "System Volume Information" (where your system restore points live) for instance, is not accessible, even to an elevated account, and therefore always shows as empty. Yes, you can cheat and give yourself access to those special locations, but that can open up a can of worms best left closed.