Just bought CoD4 on steam and it keys saying key code in use wtf?
This seems to be a common problem. I think its either a punkbuster problem or you gotta verify your key again.
https://support.steampowered.com/
Think you have to update punkbuster
go into where you enter your key (ingame) and press verify key, that fixes it most of the time
had this problem too, online multiple people seem to have multiple fixes but most of the fixes weren't for steam cod4 people how i fixed it is go into your library right click the game press properties, then click local files then verify integrity of game cache and I think that is all I did.
Sorted it, it was using my cracked versions CD Key, so had to go into the registry and get the original.