Computer shuts down problem

The computer shuts down almost as soon as it starts up. Then it will not start up again until I unplug it and start over. Each time, it will just shut down again. I took the side panel off and watched the LEDs on the Mother board. A green light stays on near the CPU. When I turn it on, a red light flashes for an instant at the bottom of the board, near the board slots. I tried a different power supply with no change. I tried unplugging all the power cables to the drives with no change. I unplugged and reseated the memory chip and the CPU with no change. I don't have another CPU or memory chip of the correct type to try in it. Any suggestions?