Will not power up.

I just replaced the power supply in my ATX, Celeron computer. When I press the On Off button the system will attempt to power up for about 1 to 2 seconds and then go off. There seems to be power to the motherboard because the Power to SIMM LED is lit. The switch is attached to the motherboard by two small wires. Could the problem be the switch or did the old power supply cause damage to the motherboard? I tried disconnecting all of the drives. The results was the same.
Any and all help appreciated.