Pretty much set up your xbox to the same connection as your PC so that you can connect to it via PC. The site above details in a much better way. :P
XBC is like a big LAN setup. Think of it like your living in your College dorm and you hook up to the network, you can play with anyone on campus! Same thing except its anyone in the world, who is also running XBC of course.
Halo 2 PC is a blast, not quite the same experience as XBL but still cool. Not to mention you can link your gamertag and get achievements as well!