Bungie has only announced it for the Xbox 360. It is so extremely unlikely that it will be backported to the original Xbox that I am inclined to call this an impossibility. I doubt the original Xbox's hardware could handle the game at all, and I doubt Microsoft, Bungie, or anyone else besides someone determined not to buy an Xbox 360 would be pleased by a version of the game that could. Halo 1 was ported to Mac OS X and Windows by Gearbox; a port of Halo 2 for Windows Vista has been announced. So it is at least conceivable that at some point Halo 3 may be ported for some PC operating systems; however, it seems likely that it will remain an exclusive on the Xbox 360 at least long enough for it to function as a system-seller for the console. Source: Halo 3 Announcement