Have you tried using "VGA Mode", also in the F8 menu? From there you should be able to reset your video settings and get your image back, at which point you can reboot normally and set your desired resolution. If this doesn't work, try rebooting in safe mode, uninstalling your video drivers completely, and rebooting with the default Windows drivers. If that works, you can try reinstalling the latest drivers. If nothing else works, try a Linux boot CD (live mode, no install required) and see if your video is correct there.