You won't be able to view/use your xbox through Media Center as it only supports set top box inputs (where it can actually change the channel on the STB). There's no plain "AV in" channel. Also, Media Center needs specific types in input cards - ones with their own MPEG2 encoder built in (your's probably doesn't).
You might need to look at an app called
Dscaler (the app - version 4, not the decoder - version 5) - it should allow you to use your AV inputs within it (instead of using the MCE interface).
That's about the extent of my knowledge on the matter, hope it helps.
Justin