That's the setup I have, so this should be easy.
1. Go to: Control panel (classic view)
2.click on "network and sharing center"
3. click on "manage network connections" in the bar on the left of the window.
4. select your internet connection AND your LAN connection
5. right click on either one and click "bridge connections"
6. when it's through with that, plug your xbox in to the Ethernet point.
7. turn on your xbox, and connect to live normally.