Ok...
The situation occurred while a game program was on - Bridge Baron, which has a fairly minimal graphics and sound requirement. Computer had seemed a bit slower of late, but no other oddities that I noticed.
Computer has been in use fairly constantly since purchase in November 2002. Operating system is Windows XP, not sure what service pack. I have been doing the recommended updates from Microsoft and had installed the latest update in the morning. No other recent changes.
Not sure what a PSU is...power source unit?
Thanks much for your help.