If you don't install the game through Steam then it won't touch it. It only updates games you bought through steam.
Acleacius wrote on Sep 8, 2011, 17:34: I want to start a Steam account to buy/play Dead Island and could really use some help. I'm concerned Valve's Steam software will have an effect on my nonSteam/nonValve games.
Does Steam/Valve scan and force update/patch all my games even if they are NOT on or purchased on Steam? I've heard it's possible to opt out of sharing data, but not forced updates. E.g. I don't want them monitoring and updating my Fallout 3 (retail version not Steam) install to breaking my mods, since it has nothing to do with Steam.