More unplanned problems. Basically, vBulletin is awful software and was creating endless temporary files which eventually caused Windows to fall over and die. In fact, I still can't delete the temp folder because Windows chokes on it!
The great news is, our backup system was working just as intended, so nothing should have been lost.
The bad news is, Windows hosting sucks, our Windows host sucks, and PHP sucks even worse than those. Maybe something we can do about that.
See you at GDC!