if you registered you game, you can look it up on impulse. simply right-click the game, and select the view registration option. it even lets you copy it to your clipboard.
however.. if you registered you game, then you really dont need the CD or CDkey anymore, (cause you can install inpulse on the computer in question, log in, and download the game) so chances are you prolly didnt register... in which case... i cant help you. (But someone from support might be able to... might) (it might be hidden in some file or folder on your computer... but that would require fun searching around, that i dont feel like doing for you... dont forget to check hidden folders... expecially appdata... where all the saves and stuff are)