Computer Post’s, No Video Shows Up/Windows Does Not Load

I’m trying to diagnose a system here. The computer powers on and posts, i am able to see the bios but after that i just get a black screen, i’ve verified that the monitor works as the light turns green, i’ve even tried using a different video card but to no avail. I’ve tested and even replaced the power supply with a new one and still nothing. The only time i can see text past the bios screen is when the hard drive is unplugged. Could it be that the hard drive has gone bad or could it be another component that is failing? I would appreciate any input as to what you think it might be.

Thanks.