If you need to return your Game back to the exact way it was when you first installed it, here is how.
Basically all you need to do is rename your game folder and let Steam recreate a totally default installation.
You do this by exiting from Steam, then go to (for example):
C:Program FilesSteamSteamApps\condition zero
and rename the condition zero folder to this:
C:Program FilesSteamSteamApps\~condition zero
When you start up Steam and click on Condition Zero in the games menu, Steam will quickly create a default installation which should be exactly the same as when you first installed the game.
You will need to re-configure your key binds if you use any custom settings.
If that all looks fine then you can delete the ~condition zero folder.
If you have any custom content that you wish to keep, you will need to move it from the ~condition zero folder before deleting it.