You're sure that the core isn't cracked? Are you sure the PS works? Does it turn on? You can test the PS by shorting pins 14 and 15 I think. You might want to search the net and make sure the pins I'm talking about are correct.
Anyways, if you find that the power supply works, then need to make sure the CPU works. Try testing the CPU in another board. My guess is that there is a short somewhere that is preventing the mobo from powering up. Also, if this is installed within your case, are you sure you have the power jumper on the correct pins? I fixed this guys computer once and turns out he had put on the jumper wrong. Took 2 minutes.
Try starting the comp with monthing else but the RAM CPU and Vid card. That's all you need to test out this system anyways, so for now don't have any other thing plugged in the PCI slots
Hope this helps. I'll try to think of some more stuff to help you out.