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)