Dimension 9200 POST problems

Hi, my dimension 9200 has been sitting unused for some months, and upon returning to it today it will no longer boot.

When the system is turned on, all fans receive power and spin as normal, the hard drives spin up, and the dvd drive is also powered. However the monitor remains in power save mode, and no sound is heard to indicate windows is loaded. 

I have tested the monitor and its cables on another system and it is working fine.

In an attempt to solve the problem I have tried the following - 

Disconnected all peripheral devices.

Disconnected all hard drives, the dvd drive, and the tv tuner card from the motherboard. At this point the system gave a series of beeps on boot (1-3-2), and the front diagnostic lights 3 and 4 were illuminated, both of which seem to indicate a memory problem.

Removed and reseated both memory modules, trying them both individually then together. This then stopped the beep codes, and the diagnostic lights show 1-2-3, but still no monitor output or signs that the system is booting to windows.

The new light pattern doesnt indicate a specific error if i correctly understand the manual, so i reconnected first the original 2 hard drives and dvd drive, followed by the 2 non-original hard drives. None of this had any effect.

Removed and reseated the video card with no effect.

Used the clear CMOS jumper with no effect.

So to summarise I have removed and individually reconnected all of the components besides the processor, PSU, and the motherboard itself. I dont believe the PSU to be the problem as everything appears to be receiving power as it should. The only non-original components in the system are 2 additional HDDs, one SATA and one IDE connected via an adaptor, but these don't appear to be the problem, nor are they the disks on which the OS is installed. Currently, turning on the PC results in all fans starting up, including the video card fan, HDDs spinning up, and the DVD drive receiving power, however nothing is displayed on the monitor, no sound is heard, and a single press of the power button will turn off the system, which i believe implies that windows has not yet loaded. Lights 1, 2 and 3 are lit on the front panel.

If anyone has either ideas what the problem component is likely to be (motherboard or processor maybe?), or how i might find out for sure, or else a solution, I would be very grateful.

Thanks,

Adam Edwards