Cannot get machine to power on

My machine has been shuting off intermittenly. It got so bad that themotherboard was not turning on. So I thought the board was bad. I bought another baord and while the board does turn on, neither the monitor nor the keybopard/mouse are working. So I hooked up the PS3 to the monitor and the monitor does indeed work. I then took off the video card, and hooked up a vga cable(as the motherboard had on board graphics) and the monitor/keyboard/mouse still would not work. So I took out the memory and the motherboard did not beep to indicate that the memory was not in place. I then took the CPU off the board and nothing worked.

Since I isolated all the parts,the only other component left is the power supply. I am thinking that is the problem. What does everyone think?

If the only thing plugged in was the MB, no ram/cpu, and nothing posted check all connections. If you still have nothing Id say it is the psu than.