Can't ativate Halo 2...

I recently got Halo 2, installed it correctly but, when I try to activate my serial key, it fails to activate it. Says that there is a problem with the servers or that they’re temporarily down. Any help would be much appreciated. (:

Me pasó algo similar, call to support and request a new serial

there is nothing wrong, if you have another Games for Windows LIVE supported Game. use that key to activate the game. it worked for me.