http://www.xbconnect.com/xbchelp/network/
I think you are using setup E.
Step 1: Plug the USB Cable that comes from the Wireless Adapter into your PC's USB port. This should already be done, if you have a wireless router for your current internet connection.
Step 2: Take a RJ-45 Crossover Cable and run it from an installed NIC card on your PC, directly to the Xbox.
NOTE: A regular RJ-45 Cable can be used instead of a USB Cable for connecting the Cable/DSL Modem to the Router. This type of network setup also requires the use of WinICS (tutorial found here) or a program such as WinRouteİ (downloadable from the world wide web). Consult the manual for specific instructions on how to set this up or visit our WinRoute Configuration section.
That's it! Your Xbox is now on the same network as your PC.
Your best bet for help would be to post on their forums. They can help you out a lot better than we can.