How do I recover my Halo PC product key?

I'm a member of the clan who owns my favorite server. Well, my friend got into the server (using my copy of the game, as he always does) and started trolling. He managed to get me banned. My only solution is to reinstall the game entirely and see if that fixes it. Unfortunately, I lost my product key somewhere and I can't find it. The only thing I can find on the Internet is a solution requiring this "regedit" program. I tried that and it's too complicated for me. What's a better way I can recover the product key? I don't want to call Microsoft. Also, based on what I can find on the Internet, I can't buy the game in stores anymore.

Thanks for answers.