System drive, OS is Windows 7 x64 SP1 Ultimate.
Windows Explorer shows 100GB of used space.
TreeSizeFree shows 75GB of used space.
Total Commander and FAR confirm TreeSizeFree's measurements (counting hidden files).
Shadow copy is using up to 5GB with the current usage of 1.36GB.
Running FAR from Win7 PE shows the same.
Checkdisk shows no bad sectors. Disk is an OCZ SSD.
SMART monitor suggests SSD condition is perfect.
Where is the rest of space gone?
Most Likely Problem: There's a large number of directories and files that even Administrator accounts do not normally have access to, especially in the ProgramData and Windows trees. Without doing something special, most space analysis programs aren't going to correctly account for the space they're using.
Using space analysis programs on the Windows folder is already fraught with problems anyway. The folders are densely hardlinked, and very few programs (including Explorer itself) can correctly handle that. In fact, the only one I know of offhand is Sysinternals' du.exe tool, when passed the -u flag.
If you want to know how much space is really used/free, check the properties of the drive itself with Explorer. Trying to analyze the space usage of the system drive with other tools can give you misleading information unless you know how to interpret it -- which isn't very straightforward.
Do the tools you used count the pagefile? I ran into a similar issue a while back where I was trying to account for 12GB of "unaccounted for" data and I had completely forgot about the pagefile.
