I assume when you mean you activate the game on line you mean your CD key to play multiplayer using Gamespy?
If yes! your retail copy of the game or a steam copy, isn't link to your PC the only thing would be link is your CD key to your email on Gamespy like user name and password for multiplayer unless you change it in gamespy.
Aside that you can re-sell your retail copy of the game but if you want to let a person play multiplayer you would have to give the CD key and user name with the password for them to be able to play multiplayer.
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