Your monitor goes through at least 3 different video modes during boot: POST display, Windows Splash Screen, and finally Windows itself. If the problem seems to only exist while Windows (your desktop interface with the icons, etc), then it is likely a driver issue. If you know what kind of video card you have, then you should go to the website of the manufacturer and download the latest driver. If your computer is something like a Dell, then the video card inside of it likely has drivers for it on the website of the manufacturer who made the PC. You just have to look the model number for the computer and go from there.
Another simple possibility is the screen resolution/refresh rate being set at something your monitor cannot handle or interpret....
What was the last thing you did before things started acting up?
