First off make sure you are using a crossover cable inbetween your computer and the xbox, also make sure your xbox is actually on, and having the xbox look for a connection could probaly help. You will have to manuely have to set up the connection on the xbox to use a crossover cable (most common are 192.168.1.1 for the computer and 192.168.1.2 for the xbox, this is the needed IP address) then you will have to setup a subnet mask and probaly a gateway. It would probaly just be eaiser if you went out and bought a router and two patch cables, connect the modem to your computer, then the computer to the router via a patch cable and another patch cable from the router to the xbox. Somehow bridge the connection from the usb to the ethernet (someone else on this form will probaly know how, probaly by internet connection settings) and just setting up the router to have somesort of DNS to automatically give the xbox its address. Im sure somebody else on this site knows a simpler way, anyideas anybody.
P.S. - Make sure your ISP will allow two connections at once.