Heres what I got. I have a 500 watt power supply that wont power up two different towers. The towers are pretty basic ones a P4 with AGP video card one hard drive, the other is a basic board with on board video and one hard drive, each have a DVD burner and thats about it. Nothing special or power demanding. I have a digital tester and everything test good. 20 pin, 24 pin, 4 pin power, all satas, all molexes. You turn either computer on fans spin but nothing comes up on screen. I connect my 650 watt supply in and both towers fire up. How would you test under load it has to be failing there but I dont know how to test.