Acer monitor AL1916W

My monitor, like others on this website, says "input device not supported" .
However, unlike some of the others, my computer won't even fire up.
I turn my computer on, it starts to go through the windows XP window (with the blue dots go across the screen) and then just freezes up before it gets loaded. My copmuter man put a new video card on my computer thinking that was the problem. He turned the computer on and as it was running throufgh windows xp (with the blue dots going across the screen) it would automically shut down and reboot. It kept rebooting itself for at least 25 times befoore windows and the computer kicked in. I took it home and turned it on and all it did was freeze up when windows xp (with the blue dots running) tried to load. So I brought it back to my computer man and hooked everything up (using his monitor) and it fired right up without hesitation. So I took it back home, hooked everything up (including my 19" acer) and once again I got input device not supported and it froze up. We haven't been able to figure out if its a video problem, monitor glitch or hardware problem, but this computer will sometimes run, but most of the time it just freezes up while trying to load windows xp. ANY SUGGESTIONS other than to trash the system?