My sister had the exact same problem yesterday, and while this is not an answer to your problem, she had no screen when the graphic card was installed and the tower was laying on its side on the bench.

Once stood up - on came the monitor, I was wondering if something was making contact with the back of the motherboard, (it is an old box that has been pulled apart a few times) effectively shorting something out. I haven't physically inspected her machine - so I can not confirm this.

Definitely a strange one.