there isnt one yet....because emulators do tend to take a lot of time to perfect, the machine you usually emulate has to be a lot more powerful than the original machine itself...i mean sega saturn emulation needs about 2 ghz just to run at the right speed
but i do recall reading on a site about someone who is working on, seeing as the xbox basically works as a pc he is trying to get the .xbe files to execute on the pc and basically run them like pc games which fair dues would probably work when you think of it in the long run...more can be read here
http://www.caustik.com/xbox/cxbx.htm