Ok im gonna assume your using xp so what you do is make sure both systems are on and go to control panel> network and find your wireless connection right click on it and go properties and theres a tab up the top called sharing and in sharing tick the box that says"allow other network users to connect to this computers network connection" then what will (should) happen is it will have a conflict between your xboxes local connection which it should be, and change the ip to a static something like 192.168.0.1. Now your xbox is connected your computers internet connection which is your wireless. After that i reccomend restarting your pc then on the xbox (after your pc has restarted) test the xbox live connection.
But be warned having your connection like this isnt really good you might randomly get disconnected at times from xbox live and lag like a b***h in game.
hope this helped
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