To find out what's taking up the room, open up your "my computer" and go into C drive. right click each folder one by one and click properties, record how much space each is taking up.
windows, program files and users should be taking up the most. Once you do this, go into the users and program files folders and dig some more to see what's using all your space.
does anyone else use your computer? someone storing some dirty videos somewhere?
---------- Post added 2012-11-13 at 11:27 PM ----------
also, how large is your pagefile? do you have system restore turned on?
I use CCleaner to scan for unwanted files, etc... give it a try. Just dont use the Registry fix, it's not needed.