Run disc cleanup as administrator and select all on the front tab .. There will be all the backups for windows updates held there
I've done that; that doesn't seem to be what's causing it. It's weird because when I installed something like Titanfall (a 56GB game), my space goes UP about 200GB to what it should be.
I've ran WinDirStat, and my 'unknown' section is about 220GB and I can't account for what that is.