Sure, the xbox runs a custom windows 2000 kernel. The key word in there is CUSTOM. It was designed specifically for the xbox. That won't make it any more feasible to run that particular OS. Like I said:
WinCE/.Net: Sure.
XPe: Maybe, if someone wants to do all that work.
95,98, 98SE, NT4/5, 2000, XP etc.... Dream on.
But please, someone prove me wrong. <