well I've never done this before but, I think what you would want to do is probably mess around with QOS (Quality of Service) on the router. Again I've never done this but if your router is half decent (my old linksys 54G can do it) but what you will want to do is set it up so that you have static IP addresses and that the Xbox ( say IP 192.168.1.100) has priority over the PC (192.168.1.101). If you statically assign the IPs they won't change so the Xbox will always have precedence.
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