http://www.tweakguides.com/ said:Users of Symantec's security products have recently been finding that they cannot launch Steam-based games. It appears the reason for this, as discussed on the Steam Forums, is that a new update from Symantec is inadvertantly flagging Steam games as trojans. Symantec are aware of the problem and are working on a patch, the only solution at the moment is to disable any background or automatic protection. Note that in general I always recommend (and so do game developers) that users disable all background programs - including security software - when gaming, as they are known to potentially cause conflicts and performance issues.