do you have an onboard video card? if you do, take out the video card and start up the computer..if u see everything boots of corectly it means your video card is fried.
If you dont have an onboard vid card, then your best bet is to test it with another powr supply or video card. if the same problem still occurs, then i would say its your motherboard or ram.
As you can see, your only choice is to try each component individually until you find the problem. i would start with vid card, powr supply, ram, and ultimately motherboard. hope this helps