Have you tried connecting a monitor?
If this works, then it may be the screen that's the problem.
Otherwise, have you tried booting from the Windows DVD, then reinstalling the OS?
If it's a software problem, this should get rid of it.
If it's a hardware problem, then at least you'll know.
(Bit of a hassle if you haven't backed up your data and program exe files / serials, but Vista and Windows 7 should save everything in a folder called windows.old)