I tried searching GLG20, GLG-20, GLG 20, etc... and could not find this problem here or on the forums at Steam.
When I first download Sin Episodes through Steam, the game played without a hitch.
At some point, I'm not sure if it was a change to the game or a change to my computer, a problem developed.
Near the beginning when the first GLG-20 flies over and Jessica says what it is, the sky turns black.
As long as I am looking towards the GLG-20, the sky stays black. If I turn far enough so that it is no longer in my field of vision, the sky goes back to normal.
I have tried all the combinations of settings I could think of...
I have the latest drivers for everything and all Windows updates.
Windows XP Pro (all updates)
ECS L7S72 (all SIS drivers)
AMD Athlon 2000+
ATI Radeon (catalyst 7.1)
Sound Blaster Live! (newest drivers)
I know my computer is pretty old, but HL2 plays at max settings just fine and Sin Episodes used to.
Nothing is overclocked, all drivers are official.
I will post a screen shot of what it looks like tomorrow if that would help.