From Greg Coomer of VALVe at 1:38AM PST
If you pre-ordered your copy of Counter-Strike: Condition Zero (CS:CZ) over Steam, first restart Steam then double click the CS:CZ icon in your Steam Games list to begin playing. Those who did not pre-order the game over Steam can purchase CS:CZ at any time.
Special offer extended for CS owners - If you already have a Counter-Strike Steam subscription, order your copy on Steam by April 1st and receive a special price of $29.95 for Condition Zero.
A new dedicated server release is also available for CZ; it can be downloaded at the steampowered website.