Yeah, trying a PS/2 keyboard made no difference so I just parted it out.
try this take the computer apart then put it together. I mean take out the ram unplug everything card network card power supply and so on. what this does cleans the contacts on the memory the cards etc sometimes this will fix it and then you maybe get into the bios. if that don't work then try a knowing spare power supply. once you got everything back together again. then if it don't boot up into the bios then try a knowing spare parts one at a time. then that way when you get to the motherboard then you will know that motherboard is dead or maybe the bios is dead. look on that motherboard for a brown spots on the chips on the motherboard or cards and the ram.
Did that computer was able to boot up into windows before you parted it out?