Help with Halo 2 Vista

So I recently downloaded halo 2 vista but I don’t have my Product key anymore. I heard somewhere that I can reuse older product keys. Is that true that I can use older product keys from games and programs? Could I also buy a thing of Xbox Live Gold and use that key?

If someone could help, I would really appreciate it, thanks in advance

If you have played Halo 2 on PC in the past on your LIVE account then you should be able to just login and play.

If you haven’t played Halo 2 on PC on your LIVE account before then you won’t be able to since Microsoft turned off all new GFWL activation.

You don’t need Gold to play on GFWL.