Figured it out.
It seems that on all the Power Supplies theres a metal casing that goes around it, kind of like a giant heatsink. Well, MS decided to solder that metal casing directly to the board in 3 places (i think it also acts as a ground, as one of the solder points is right next to the ground wire for the plug). And in removing the power supply from its housing, those solder points were pulled out, on both the new power supply i got and the old one i have, which is why i get a 3 red light error when i use the old one.
So, while i dont want to have to buy a whole new power supply (but i will just because i want to get this mod done with), what could i possibly do to fix the solder problem (no traces where lifted, only a small peice of the PCB board around each solder point)? Is this easily fixable?