Ok I'm stumped.

Plug it in press the button. Not much happens. Fans turn on but, Display does not light, No post, no splash, no beeps. Blindly hitting f10 or f anything does not help.

The dvd drive light blinks, the tray opens and closes, but a dvd's inserted don't produce a boot. Ubuntu included.

Tried plugging in an alternate hard drive, no change.

Next step: unplug/remove as much hardware as possible. Drives, network card, RAM, etc.

Try again.

Then add back one stick of RAM.

Tray again.

hmm
wuff- this review seems to think that the cooling is a problem with them. Could the CPU be toast?
Are all the fans working and heat-sinks clean?

Yes when stressed the fans go into "Turbo" mode. It runs hot most of the time.

I sent an email to HP service about the problem describing all the trouble shooting steps, swap drives, reset cards and connections, try a different monitor...

Their response is that the mobo is fried and that I should visit a computer store near me to purchase a new MB with the following form:
* mini-ITX form factor
* dimensions: 19 cm (7.5") x 17 cm (6.7")

The machine was unplugged since last night. Tonight It booted.

After about 90mins. Of software updates and futzing around. Core Temp is reporting 64C.

Started an anti-virus scan which is using approx 50% of cpu time. Temp has jumped to 75C.

Playing multiple YouTube videos pushed the cpu to 100% and the temps to a high of 79C

I pulled the pwm wire from the cpu fan's connector (4 wire fan), forcing it to max rpm's.

The cpu stays below 45c now. But the fan is obnoxious.