Halo 1 was originally a PC game
Halo 2 was exclusively for Windoze Vista, but later ported to Xbox.
Halo was originally a Mac game in fact. Bungie was a Mac developer (they are famous for the Marathon and Myth games) and was major force to be reckoned with long before Halo was released. It's why Microsoft bought them.
Bungie single-handedly carried the fledgling Mac gaming industry and nobody remembers this which is a real shame. They are the fathers of Mac gaming.