My Counter Strike Doesn Work

[Conqerempires]

My Counter-Strike doesn't work for some reason. I dont know why it says this message everytime I go to a game. It says "Your Cd key cannot be used on the VALVE system." How do you fix the problem? Can you please tell me with detail how to fix it? Thanks for those who replyed back.

[Droppyale]

did you buy the game?

do you have a legit cd key?