It sounds like a video error. First make sure your drivers are up to date, and Windows Update hasn't swapped in the crap Microsoft drivers for your correct ones.
You can try this video fix -
viewtopic.php?f=13&t=347You can also try adjusting the resolution or running the game in a windowed mode:
Edit My Documents\Dungeonbowl\config.xml (make a backup first)
Look for the <Video> section
Try changing the resolution to something generic like 1280x1024
Try setting Fullscreen to 0 (windowed mode)