Hmm, I haven't ever run into a computer that wouldn't boot because of the hood sensor / chassis intrusion thing. I only ever saw it where it would display a message when the computer booted saying the case was opened. But it all depends on how the individual PC's BIOS was configured, I suppose.
When you say the power light is on, is that after you push the power button, or is it always just on as soon as you plug it in? If it's when the power button is pushed, I'd say the button is connected wrong. If the pins aren't labeled, try disconnecting all the wires and using a flat head screwdriver to jump different pin pairngs. If it boots up, that's the pair to attach the power button to. The rest (reset, HDD activity) aren't essential, really.
You also mentioned it's not in its original case. Is it in any case? If it's just sitting on a desk, it could be a grounding issue. Try working on it with it on a static shield bag, or reinstalling it in a case to reconnect it to the ground. Haven't experienced it with a MB, but have with a HDD.
Also, not sure you'll get the best computer advice on a personal finance forum. Yes, it's General Discussion, but I've found this section of the forum to be pretty slow compared to the rest. Might want to try another site specifically for computers. I like bit-tech
, based out of the UK. Full of helpful people and active forums.
Good luck, and let us know how it goes!