I'm not familiar with the m8100n at all, but what it sounds like is that it has a tv tuner card built into it. Do you have xp media center or the equivilent version of vista? If so there should be a neat little media center button somewhere. Either in the quick start bar next to the start button or somewhere in the start or programs menus. If you just have a normal version of windows (which would be weird if the computer came with the tuner card) there should be some other utility that's used to access the content on the tuner card. Once you get either of these up, you'll have to look around for either a source search or source switch button. It might "just work", but I wouldn't bet on it.
It might also be easier to just plug the xbox into the monitor if it is a multimedia type, but I wouldn't bet on that either.
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