Go to My Documents\Penumbra Overture\Episode1
and change the following parameters in settings.cfg
<Graphics NoiseFilter="false" Bloom="false" MotionBlur="false" MotionBlurAmount="0.150000" DepthOfField="false" TextureSizeLevel="2" TextureFilter="1" TextureAnisotropy="1.000000" Gamma="1.000000" FSAA="0" PostEffects="false" ShaderQuality="-1" LimitFPS="true" Shadows="0" />
Everything will look like shit, but you'll be able to start the game probably and then you can muck about and try to fix it.
Worth a try at least