Since the last batch of Microsoft patches, (may not be the cause) I am seeing this message in Event Viewer on boot up. It appears not as an error or warning, but as an event about 5 times in a row. It's descibed as "Application Popup". Examining the event shows this description: "Machine Check: Regs"

Thats all it gives. Event ID 26

Googling it gives little help, but seems to point towards a hardware error. I doubt that is the case.

Any clues out there?

OK, here is the answer to this. (Let the next Googler find this clue)

This computer has an APC Backup 350 connected by USB for power monitoring. The smart chip in the APC has gone bad and is sending garbage along with valid messages. Disconnect it and all is well. Isn't that event viewer message helpful?

Edit: The error with the UPS was real but not the true cause. The problem returned the next day. I ran extensive diagnostics on this machine and it passes. Also the machine is rock solid - never crashes. So for now, I'm going to live with it. By the way, this is an Athlon XP processor and one person reported that AMD said it's not a cause for worry.

Hhhmmmm, see my answer here: http://www.msfn.org/board/index.php?showtopic=39298