Key code don't work

I bough battlefield caompagny 2 and was able to install and play solo and online.  I had to replace my hard disque because i had a probleme with it.  I reinstall the game, i'm able to play solo but when i try to play online the system ask for the key and when i enter my key the system respond by the key is already in use.  This is probably normal because i use it on my previous hard disk who is defective right now.  How can i fix this problem?