Foxconn 760GX K8MC-S

I changed the power supply and put a new one which is 500 watts. I plugged all the cables the same way they were plugged with the old power supply. I then turned on the swith from the power supply the power light came on and the power supply fan and processor fan came on and the motherboard starts to boot up for about three seconds and then it goes off. I turned off the switch from the power supply and turned it on again and it does the same thing. I hit the power switch from the case to see if it comes back on but it doesnt. I unplugged the power switch and turned it on with the power supply switch and it does the same thing it goes on for a few second and then turns off. I cleand the processor fan to make sure is not an overheat situation but it's not that. What can I do to to correct this problem. I have check all the wires, removed the led, hdd, and reset switch and still nothing, what can I do or check that I have'nt done yet ? Thank You for your help.