When you go to start your game, if you can, go to options, video, and then change it to whichever option is not highlighted.
If you have no other option, open you FeralHeart folder, and find the "ogre.cfg" file and open it in notepad. The first two lines should say something like this:
Render System=OpenGL Rendering Subsystem
[OpenGL Rendering Subsystem]
If it looks like that, change the "OpenGL" to Direct3D9. If it says "Direct3D9" change it to "OpenGL"