Then I turned my attention to the Exchange server (which had an almost full c:), and found that the root of the problem was McAfee GroupShield (our email anti-virus software): it had created over 49,000 log files sucking up 7.3GB of disk. Ouch. That was something like 50% of the drive.
TreeSize is a really nifty, lightweight GUI tool for finding folders that are hogging all of your disk. Highly recommended.
I came, I saw, I deleted. And I feel much better.