Have you tried this?
"
Originally Posted byZanyCynic
You rightclick the game, select Properties, deselect "Enable Steam Community In-Game".
It is per game since the new UI, so it is perfect."[DOUBLEPOST=1356694406][/DOUBLEPOST]and
"I had this problem as well, but I tried running Compatability under Windows 98/ME and disabled the Steam In-game community by right clicking properties.
Problem seems fixed and I can left click to move again, the only downside is the stuttering sound while loading a level (is fine in the level).
But yes, anyone having these issues should try running it under Win 98/ME AND disable Steam in-game community. Hope this helps anyone!"