Could be issues with your video driver, make sure its up-to-date and/or reinstall it (prolly from safe mode would be best for that).

If its only happening on bootup, its likely some service/application starting up thats causing the issue. I'd suggest downloading AutoRuns (AutoRuns for Windows v8.73) and try disabling some apps/services running on startup to see if you can pinpoint what is causing the problem.

Note: If you are not really computer saavy, take your time with disabling things and KNOW what you are disabling (Google will be your friend for looking things up).

HTH.
Rick