Mobo or Proc Problem

After eating lunch yesterday, my husband helped me in checking our broken computer. It was not turning on so I thought it is the power supply. What I did was I removed all the cards (video and modem), disconnected all the drives (hard drive, floppy and DVDRW), disconnected power to the processor, take out the memory, leaving only the mobo and power supply. When I tried to turn it on the fan on both the power supply and proc will spin for a few seconds then would go off. I am already thinking of getting a new power supply. Our neighbor says they have 2 computers at home that currently they are not using. My husband took it so that we can test the power supply but then both computer is using a smaller plug. Their power supply is only 20pin while ours has 24 pins. Good thing my husband remembered that her ate also have a computer. I am not sure what is the problem of that computer but that computer does power on. The first that we checked is if it is using 24pin. Yes they are compatible. What we did first is we connected the working power supply to my computer and it did the same thing. It spin for a few seconds then go off. We connected our power supply to that computer and it worked fine. Telling us then than the power supply is ok and something is causing it to stopped. The only thing left inside the computer is the mobo and processor. I wonder which is bad. I guess it will take a while before we will be able to fix it. I have a lot of  bills to pay. I still need to save money to replace which ever is bad. For the meantime goodbye Frontierville, goodbye, facebook, goodbye ikariam and goodbye DOTA. @_@