Sounds like a definite hardware issue to me. Just to make sure, though, does it flash from the moment you turn it on (at the logo screen and while Windows is loading), or does it only start flashing once you boot into Windows?
Assuming it's doing it the whole time (which I suspect is the case), it's a hardware issue that isn't going to be easy to fix. It could be the screen itself (or various components of it), or it could be the video card. It probably just depends on what the laptop is worth whether you want to try and fix it.
If you decide it's not worth fixing, you can always buy an external monitor and turn it into a desktop.
Sorry that I don't have any great news for you.