Here's a thought, are you running any fpeer to peer program (like Kazaa). On my server it's straight Windows XP and Kazaa on a 4GB hard drive. I found there was like 2MB free after awhile, and could not figure it out. I tried clearing tmp files, even went and cleaned out all the temp directories of all the users on the computer (which is a another thought, plus the standard windows directory temp folder). Found out it was the Internet Temporary files that had build up to the point where it was the rest of the free space. So it's something to think about checking on just incase.

Just a helpful thought...
~AciD