dell dimension 4100

Hey everyone, not sure what to do from here - I've got a dell dimension 4100 desktop with a 97ujy motherboard. For some reason it will not post - I've checked the manual and the error codes (from the lights at the back of the motherboard - none of the lights are on - or at the front). On the manual it said that if the lights are not on at the front or the back then it is the power supply (which is odd because the fans work). But I've tried switching it out with another power supply that I know does work from another computer and the strange thing is that when I have the old power supply in both the power supply fan and the cpu fan come on, but with the new power supply neither one comes on. Any suggestions?