Disk space mystery solved!
After spending hours trying to find 20 GB that Explorer reported was on my C:
drive that wasn't there when I added up the sizes of all the visible and
hidden files and folders, I finally discovered 16 GB of data in the
C:\ProgramData\Microsoft\Windows\WER\ReportQueue folder! 16 GB! This folder
is the Windows Error Reporting queue. I have been having some trouble with
Windows Update and a few other Vista things, but my problems can't be so
interesting that it takes 16 GB to describe them.
I only found it with a freeware program called WinDirStat. Explorer didn't
show it in any folder.
I just deleted it all and so far so good.
If you're having trouble finding what's sucking up your disk space, check
out this folder.