Halo PC halp

Alright, so the version of PC I had shit the bed (screwed up visuals and messed up my computer when I would exit). I didn't know how to fix it, so I shitcanned it and DL'ed another version. To play it I have to register the game online (I assume this is where the product key comes in), but when I click on that option in the game's menu, it takes me to an invalid webpage and I can't do anything. wat do?