Computer acting up?

I found a computer on the sidewalk, since my Video Editing Computer was down, having thermal overheating, I took this home and switched out Motherboards. I an now running the computer with the motherboard and Hard disk from the one I picked up. I downloaded the instruction sheet for the motherboard, made sure it was all hooked up right. Now all it does, when I press start, is run the fans for five seconds, shuts off for 10 seconds and then runs again for 5 or 6 seconds, then repeats like that. What can do to get this to run properly?