I'll help, ive done what you want to do, saves yourself£60 on the wireless adapter.
Obviousley join the xbox to the PC via the ethenet cable, then on the land connection you need to 'ctrl' and click both and then pair i think it is or bridge. ANd its as simple as that. Obviousley your PC needs to be on in order for your 360 to have internet access. I had a thread on AV Forums about this, ist agood place to try, had a larger gaming section than ocuk.