To play Halo 2 on Xbox Live, you will need a current Xbox Live or Xbox Live Gold account. To activate the hi-res modes of Halo and Halo 2, for Xbox Live members (either Gold or Silver members), just connect your Xbox 360 console to Xbox Live and insert an original Xbox game. The system will automatically check if you have the latest emulation software, download it to your Xbox 360 console as part of a system update if you don't, reboot, and start loading your original Xbox game.
Those who are not part of the Xbox Live network will be able to download the emulators from Xbox.com, and burn them onto a CD to insert into your Xbox 360 where it will auto-install the emulator. Or you can go to Xbox.com and simply sign up for a CD to be sent to your home for a nominal shipping and handling fee.