Halo CD Key recovery

Any suggestions on how I can recover the CD Key for Microsoft Halo PC game. I bought the game for my son for Christmas. He affixed the label to the laptop for safekeeping and it has now faded. We don't have a problem with the game now, but my daughter's laptop recently crashed and I know all too well of the problems that are incurred without the original CDs and product keys. I am trying to create a file of all our software product keys for future emergencies. Any assistance will be appreciated. Thanks!