ARRRGH!!! I just did another render (a little shorter than the first) and the same thing happened. HELP!!!

Thanks for the suggestion. I just did a defrag of all my hard drives a few days ago. And disc cleanup shows that there are very few files that could be removed.

Oh, one thing I realized. The quality in the scene doesn't change until I try to close the error message. Then it changes the image onscreen to a lower render quality and also changes the settings in the actual render settings menu. (!?)