My PC has decided to only display a black screen when I boot it. No motherboard splash, command line prompts or anything but it is black as the monitor will turn off if there is no signal. It does power up, do the 4 bleeps that indicates all is well and powers anything plugged in to it. I've reset the CMOS which has taken as the fans are now spinning up to max rather than the custom settings I had before but it hasn't fixed the problems. So I'm assuming a bit of hardware is dead. Here is what I think I've ruled out:

Graphics card: tried two with the same result
Display: it displays the graphical corruption the 2nd card has perfectly
Hard drive: Should still boot and then moan in past experience when I haven't plugged it in properly
Power supply: It turns on and everything seems powered

So that leaves motherboard, CPU and memory. Any good suggestions on how I work out which of these three it might be? Or if I've wrongly ruled out one of the others?

Thanks for any help.