This is an automated message generated by Steam account administration.
Steam has detected that your copy of Condition Zero has already been registered to another Steam user. Your product's CD-Key (the number printed on the CD case which you were asked to type in after installing the game) matches one already in our database.
You will not be able to play Condition Zero unless you provide an unclaimed CD-Key. To provide an unclaimed CD-Key, you will need a copy of Condition Zero which has not already been registered on Steam. Double-click on the name of the game in Steam's "Games" window and then follow the given instructions.