What make and model computer is it? Are you using onboard video, or an add-on video card? Have you made any hardware changes to it since you've owned it? Has this problem been getting gradually worse?
BTW, to cut the power when you've tried to start it and it won't complete a normal startup, try holding the power button for about 4 seconds, maybe a little longer; that may turn it off and you won't need to unplug it.