You can try cleaning up the CS:S GFC files and redownloading via steam
Goto \Valve\Steam\SteamApps and delete
counter-strike source client.gfc
and
counter-strike source shared.gfc
then download via steam (right click on CS:S and click 'update')and see if that fixes it. It will be a long download.
However, if there was some problem and CS:S need to be 'fixed' via steam by a download are you sure the download was finished?
Did you check the 'monitor' like I suggested? Because the only time I heard on the problem you are having was when folks tried to play the game while it was still dl'ding.
Check the monitor and see...