I'll guess. There are a number of low end video cards which will report they can run the shader level the new features need to work and they can, but not in real-time.
If the "Detailed Terrian" option is available, and when you turn it on your frame rate tanks, or locks up the computer, then do not turn on the feature.
That is a general answer. To get more specific, I would need to see a DXDIAG output.
