I played through the first 5 episodes with no problem with a Radeon video card. I downloaded 400 days and the launcher gives me the error:
"Could not initalize Direct3D. Make sure you have current video drivers and close any other applications that might be running. You will need a video card that supports at least pixel shader version 1.2."
I installed a nvidia 660 in January and that's the only thing that I can think of that might be causing a problem. Someone please help.