PC wont boot- no video output

We have an Aspire T310 desktop with a P4 CPU

When I power on, the fan revs up to max and the HD LED flickers but the machine does not provide a video signal and from the sound of the HD, Windows is not booting. There are no HW beeps signalling an error.

I swapped out the power supply and tried both the HD and the memory in another machine. All are fine. I also reset the CMOS memory using JUMPER JP1. I also tried booting without the HD and CD connected to the motherboard but the Acer logo does not appear on the LCD screen which works fine on another machine.

I guess there must be a problem with the CMOS (corrupted), the motherboard or the P4. Can you help me narrow down the problem without swapping out the P4? Is there a test point I can use to identify the problem?

Many thanks

Checks:
no splash screen, no BIOS screen, no beeps
Fans- both, rear spining fast
LEDS- power on, flashes twice
HD and CD-ROM LEDs flash, CD drive can be opened
Reseat- reinserted memory
Changed PSU
Reset CMOS memory (jumper JP1)