Why can't I play Halo for pc online?

Do I have to download gamespy arcade to play it online?

No, Halo for PC should come with its own browser.

Cause #1: What's the message that displays when you try to access online? It's possible that your firewall configurations are not allowing Halo to access the net.

Cause #2: Or, even worse, someone else could already have your CD key (gotten illegally from a random CD-key generator) and is using it to play multiplayer. Because Halo will not allow multiple uses of the same CD key for online play, if this is true then there's nothing you can do except download the Halo demo (but that only lets you play online on Blood Gulch and the lag is horrendous).

I hope Cause #1 is your problem, because if Cause #2 is true, then there's nothing you can do. :( Someone has already stolen online play from your game.