Turns on, then shuts down before anything shows on...

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?
mine shuts down 2 secs after being turned on and after will not let me turn it on for while after but then it repeats itself all over again...this all happened when i decided to clean it out because it was getting dusty on the fan...but instead i decided to try out another i had which was made for amd like the other i had and both made by avs and both looked bog standard so i tried itbut then after putting it back together nothing at all like i said previous just keeps turning off...ive tried to change it back to old but it does the same thing let me know if ya think you have solved it as im on a tight budget these days and replacing something i don't need to will only serve to piss me off....i reckon its either the switch or power supply but im not sure but could be motherboard or ram or battery who knows...maybe you do...cheers