What's your best guess?

My PC won't start! I was checking the RAM with Memtest 86+ (v.1.70)
while I watched TV. When I looked back at the computer at some point,
the monitor had gone into sleep mode. Nothing would wake it. I
noticed there was no HDD action. When I tried to reboot using the
restart button, nothing happened. The main power button wouldn't even
work, so I had to turn off the power switch on the surge protector.
When I switched that back on a few minutes later, the PC came on
without my doing anything else. But it wouldn't post. The power
light on the case front was on as was the indicator LED on one DVD
burner and both LEDs on a card reader. All fans were running. But
there was still no signal to the monitor. But at least the power
button would work, although the reset button still wouldn't.

All four RAM modules checked out fine in another PC this morning, so I
figure that probably narrows the problem down to the motherboard or
the PSU. When I turn the computer on, the power indicator light on
the front comes on as does the one on the board itself. All the fans
come on. There's no signal to the monitor and no indication posting
is even started. There are no obvious motherboard problems such as
popped capacitors, nor are there any obvious PSU problems.

There have been a couple of occasions over the past few weeks when
posting started when I turned the computer on but then froze while it
was polling the drives. Each time it booted without problems after I
restarted it.

The board is an Asus P5P800 SE. The PSU is an Antec TruePower 2.0

Any guesses on what is probably wrong? Suggestions?

Thanks for your input.