How can I tell what's taking up so much disk space?

Since you've already done the usual things to free up disk space, we'll start with a couple of things many people don't try.

In a command prompt, run:

chkdsk /f

That will probably tell you that the drive can't be locked, and would you like to run it on next reboot, to which you would answer "yes". Reboot the machine - as part of the reboot, it will now run a thorough scan of the hard disk and correct any logical errors that may have been introduced. Quite often these types of errors can be introduced when a program, or your computer, crashes.

Now, I'll warn you that if the disk is really corrupt, whatever the cause, the result may be some lost data as well. I'd definitely recommend backing up or copying off anything that you'd consider critical.

But my motivation in mentioning chkdsk here, is that it's also possible that the process will free up a bunch of disk space.

Now, as to identifying where all the disk space is being used, go download the diruse tool from Microsoft. Using this tool we can find out where, on your hard disk, the most space is being used.

After installing the program in the default location, in a command prompt, run:

cd "\Program Files\Resource Kit"
diruse /s \ | sort

This will take a little while to run, and then all of a sudden a list of things will come flying by the screen. When it finally stops, the end of the list will show you the directories that are taking up the most space. For example on my machine it ends like this:

  876171666   7065  \i386
  959769713     14  \Program Files\Microsoft Streets and Trips\Data
  Size  (b)  Files  Directory
 1063624662   1127  \Program Files\Microsoft Visual Studio .NET\Msdn\1033
 2684179043     13  \
30070896690  135246  SUB-TOTAL: \
30070896690  135246  TOTAL: \

Ignore the size, total and sub-total lines. The biggest directory on my machine is the root directory (just "\"), which contains the system swap file, and the next biggest is the "Msdn\1033" directory within my Visual Studio installation, which contains the on-line product documentation. Your list will, of course be much different.

Once you see which directories are the largest in terms of used space, perhaps that'll give some clues as to what you might be able to do to recover it.