What's using up my disk space?

"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"

You could alternatively try JDiskReport to analyse your disk.

There are all sort of reasons why disk space can be overused, and this is a job that System Admin people have to deal with all of the time. One common reason is the Virtual Memory page file, which is related to how much RAM you have. In the end, it might be a bunch of log files buried deep somewhere in folders, or a hidden "media" stash, or whatever, so a tool to visualize the whole drive is very useful. My tool of choice in this regard is spacemonger, but there are others.