Well there a few things you can do.
First off do you have high speed internet, cable or DSL, T1, T3, OC-48 (LOL I wish <evil grin>)?
If you, you can buy a wireless router hard wire you XBox in to the router and put a wireless access card in your PC.
If you don't have high speed internet, just get a wireless access point and a wireless accesss card for your PC. Hard wire the XBox to the access point (after you have set it up with your PC) and install the card in your PC and it should be done.
I my self use a wireless router and have my XBox wired to that and all my other PCs have external USB access points. Oh all my goodies are made by
Belkin.