Some things you can try:
1) Delete your user.cfg file to get all of the settings back to their defaults. Where this file is depends on your OS.
2) Update your graphics drivers
3) Update to the latest version of DirectX
4) Play in a window instead (this involves changing the r_fullscreen setting in user.cfg to 0).
Depths of Peril, Kivi's Underworld, Din's Curse, Drox Operative, Zombasite, & Din's Legacy