My computer comes on but there is no video display. The two fans work and the light on the "on" button lights up. There are no beeps when I turn the computer on. I can hear the hard drive spinning and the cd rom tray opens and closes when the button is depressed. The computer is a Dell Dimension XPS T600r. All connections are secure. The monitor is not a problem because it works fine with my other computer. I tried another video card but the same problem. There are four small led lights on the back of the computer labeled ABCD that do not light up. What could be the problem?