Halo 1 and 2 are already on PC, though I think 2's online functionality will be gone when GFWL shuts down. 1 might be dead these days.
Halo CE is still alive, to some degree, if http://www.gametracker.com/search/halo/US/ is anything to go by. It's certainly more alive than some newer FPS games; I wish the Battlefield 2142 server browser had pages of results.
I really should find that disc and reinstall the game.