I (finally *) got WinXP installed on my GA-P55-UD6 board and I left it running overnight.
When I came to it this morning the monitor had no image, and the system didn't respond to mouse, keyboard, or ctrl-alt-del.
I shut the power off and restarted, but the monitor stayed in "sleep" mode (no video signal).
The post codes would show C1 (detect memory) then 25 briefly (early PCI init) and then would show 26 for several seconds and then return to C1 and start over.
I swapped the video card out with no improvement.
I pulled 2 of the four memory sticks out and (!!) on the next boot I got video!
Now it advances to 50 (init USB keyboard and mouse) and stops there.
I have a PS2 keyboard in the PS2 connector, and a USB mouse. Unplugging the mouse did not help.
Yesterday I made it all the way through a Windows install, so I'm assuming that something went bad during the burn-in.
The fans are all connected, and the case is open. Nothing felt or smelled warn this morning.
I've built at least a dozen computers but I've never had to RMA a motherboard. It looks like this one may have to go back.
(*) it's not possible to install XP lower than SP2 on a board with PCI express. That killed several attempts to install yielding an IRQL_NOT_LESS_THAN_OR_EQUAL_TO error.
Mark