From your description it appears you do not even get to the BIOS bootup splash.
This means, your board has not boot-up even to the point of passing primary board tests.
Stripping down the machine is the best way to go.
If there is a power-overload from one of these things it will show up.
- Remove the HDD and CD-Drive
- Remove graphics cards
(does your board have on-board graphics? guess not, since you mention AGP)
- Remove all RAM sticks
- Disconnect any USB ports (leave only the Mouse and Keyboard where ever they are)
- Confirm the PC-Speaker wiring is fine (so you would hear any beeps if generated)
Now, power on and try a boot.
- Do you hear any beep?
Did it beep any time before it went bad?
(that would reassure us that the PC Speaker wiring is fine).
- You should see the processor fan spin,
but are there any LEDs on the board itself? does any thing else light-up or flash?
- If you think the board is not booting any further and all your data is collected,
Look for the RESET button (not the power button) and click it gently once.
Do you see any difference in the system?
Does it appear to have taken a reset and restarted the boot attempt?
If there is no data or sign of life at this point,
You should first suspect the Power-Supply.
Check it with meters.
After that, the next suspect will be your board power path
(look for blown capacitors).
Finally, you could get your motherboard number and update that in the question.