Not sure if this is the cause here, but two system files that i've found eating disk space are hibernation files (if you don't use it, then Control Panel - Power Options - Hibernate - disable removes the file.) and System restore points (My Computer - Properties - System Restore).
In my case the former was using up 1Gig (I never hibernate the machine, so it's wasted) and latter was using up several gigs more. You may not want to turn system restore off, but if you click "settings" you can tell it to reduce the amount of space it uses. Mine was set to 20% of the disk, and it used the lot
