I suggest that you do an 'Error check' on the disk as a starting point. Go to the properties window for the disk (right-click on the drive in an explorer window to get to it). This will do a basic check for errors in the index of the filing system.
There is also a Disk cleanup option in XP, and I guess Vista, that can clear out TMP files etc. But if it was that sudden it sounds more like a FS problem.