Xbox360 Live connection error

Hi im trying to connect my Xbox360 to a dell vostro 200 desktop it uses a peer-to-peer internet connection or has the ability to switch to an access point.when ever i use my ethernet cables my xobx does not detect my computer...ive done this many times the last was a laptop and it connetced just fine. they are both on vista (i have a laptop that used xp but something happened with the internet and it is currently being fixed)