C: drive says it's nearly full but it's not

So I'm doing some work for my grandmother on her PC and it says she has 300MB left on her 150GB C: drive. I go in a level, turn view hidden files on, select all and check the properties, but only 30GB is accounted for. Where on earth could the other 100+GB be????
Edit: adding info as I go.
OS: Windows Vista.
Chkdsk comes back saying 156xxxxxxKB total disk space, 151xxxxxxKB in use in files, but the properties of all the folders in the C: drive is 30GB.
I've run disk cleaner and checked the restore points, no luck. Checking WinDirStat now.
WinDirStat says there's about 1000000 fewer files than chkdsk says.