Guys, I have STALKER, Doom 3 and Doom 3 ROE... as I get free games with my GPU, I downloaded and installed it, see that STALKER and the two Doom games are available on Steam, and see the "Activate game with Steam" option. I click it, input my CD key, and it gives me an error saying "This key is invalid. Make sure you didn't miss type it...." any idea what causes this? The keys are legit, and install fine with the game. Steam won't accept them though.