An XBox emulator has recently appeared on the scene. What I've read
suggested it plays Halo for XBox acceptably but not much more as it's a
work-in-progress. As I don't own an Xbox or have Halo for Xbox I can't
confirm this, although I did dl the emulator itself.
Interestingly, there's a PS2 emulator that's also shown up but apparently it
further behind than the Xbox emu.