no where bro just buy the thing... sorry
Buy Halo (What ever Version you require) then install it onto your computer..
I dont know why dont you go ask the president and see how he would answer that question.
You ask someone for it, or download it from some p2p thing, but that would be illegal, and you would need to modify your xbox to play it anyways
sorry but there are currently no plans to port halo 3 to the PC, only on 360.