There is an excellent utility named windirstat that will scan the drive and tell you where the space is. For some reason, there is an option that you have to set to show "hidden" space.
I suggest that you download that utility, try it, and let us know if it helps.
Your avatar looks like it contains the logo for Shell gasoline - what is that?
'twas, an old back up file, derp.
Useful program, thanks.
The picture was the first thing that came to hand when I was making the account, it's something a friend made on FlockDraw.