Im trying to do the same thing and work work, I'm using a usb wireless adapter and the ethernet port on my motherboard, under network connections the LAN port just says unidentified network, shared. and my wireless adapter says dlink (which is my networks name) and has the full green bars, I select both of these and right click and select bridge connection and it gives me an error saying you need at least two lan connections or two highspeed internet connections thats are not being used by internet connection sharing.
After messing around another internet connection showed up randomly, wireless connection 3, not connected, and the device is called microsoft virtual wifi miniport(weird becasue i dont own another wireless device).
when i go to all network devices it recognizes an xbox 360 as a media device (also shows my pc under computer and again twice more under media devices not sure why) also shows internet gateway device and wo different wireless n routers (different icons) which is weird because i dont have a gateway device, just the dlink router and belkin adapter
I used to use my linksys ethernet adapter for my xbox but it now randomly wont see a network on my xbox just says specify unlisted network or create adhoc, I dont want to buy a new one when my computer should be able to bridge