What's using up my disk space?

My Vista 64 installation has drive C, a 150 GB hard drive. I also has a 1.5 TB drive D. My C drive has 12.3 GB free. I don't know what's using up all the space. There's almost 83 GB unaccounted for!!!

As per this question and this question, I downloaded WinDirStat and SpaceMonger. WinDirStat, once I found the right options, tells me there's 82.9 GB of . SpaceMonger, which has a particularly nice display of the data, tells me there's 82.3 GB "Unscanned", with 2 unscannable folders.

Where do I turn now? Both tools roughly agree there's a large amount of data on my drive, but neither seem to be able to tell me what.

I've tried the standard approach; disk cleanup, remove hibernation file, set the virtual memory to the D drive instead of the C drive.

It could be the System Restore service (and previous versions) eating that space up. Those are stored in the hidden \System Volume Information folder in the root of the drive. If you open an administrative command prompt window you can view the usage of System Restore/Previous versions by issuing and reading the following command

C:\Windows\System32> vssadmin list shadowstorage
(program output follows)
....
For Volume (C:)\\?\Volume{some-guid}
Used Shadow Copy Storage: 5.317 GB (3%)
Allocated Shadow Copy Storage space: 5.806 (3%)
Maximum Shadow Copy Storage space: 7.448 (5%)

If you find that they're using a large amount of storage you can issue the command vssadmin Delete Shadows /For=C: /Oldest to delete the oldest shadow copy.

You can set the Maximum Shadow Copy Storage space to 12GB with the following command:

vssadmin resize shadowstorage /on=C: /for=C: /maxsize=12GB

Joshua's answer is probably the most likely, but have you ever used Offline Files on this computer?

The client side cache (CSC) may not properly empty, even if offline files is disabled.

You'll want to delete the contents of %systemroot%\csc to clear it up (or at the very least, verify that this is the information that is taking up the space.

MS KB article on the CSC.

You could alternatively try JDiskReport to analyse your disk.