Nice thought, but been there and done that
Thanks for the suggestion, but I tend to doubt it is a heat/fan issue for a couple of reasons. I didn't mention that this is a fairly old tower unit, and we replaced mother board, heat sink, and fan a couple of years ago. The machine gets cleaned thoroughly at least once a year, and most recently did not look crudded up inside. Also, we have monitoring software that has not indicated heat problems.
To answer your comment about the registry cleaner, I read several articles skeptical of their use but decided on it out of desperation. Nothing else was working, someone whose opinion I trust said it probably would not make things worse. Hey, I got the program from cnet! Seriously, I didn't have much faith it would work, but the down side seemed minimal.
Anyone got any opinion re: whether reinstalling XP completely would make a difference?