Pls answer each of these questions
1) Did you install an addon graphics card in your computer? If yes, have you tried simply removing it?
2) Have you looked in your BIOS. Does your BIOS show settings for onboard graphics?
3) You DO see a flash screen when you power up, right? It just goes black when Windows starts?
4) What version Windows are you running? And do you have the install disks?