) Install XBC
2) Connect your xbox(360)/psp and pc (crossover/network cable)
3) Detect your xbox(360)/psp (automatically)
4) Join/create a (systemlink) Game
Go here for screenshot:
THIS WORKS 100% (I own an xbox myself and played halo2 on it dozens of times with random players from all over the world)
Play HALO2 Coöp/Deathmatch/Mods have tournaments, Go WILD!
NOTE: It's still a small community so spread the word. And help keep this thread alive!