firstly, do u have a network set up?
the computer you are on now must have 2 network cards
one connects to the internet
the other connects to your xbox (or hub, that connects to the xbox)
if u don't have 2 network cards, get two, and install it
if u have 2 network cards, set up the ICS correctly, control panel, network connections, setup a home or small office network
it'll ask u for which network card connects to the internet, which network card connects to your network
if u have that done, make sure u have a cross over cable (if ur network cable connects strait from the 2nd network card strait to your xbox) (1st network card connects to your internet)
or
make sure u have both strait through cables if ur 2nd network card connects to a hub, which connects to your xbox
if all still doesn't work, call your ISP, they might help u, or tell u a site to go to, i know for rogers cable (in canada) does.... i'm not sure if they'll rip u off or not, and somehow make u pay for a 2nd ip like they did in the past.... but basically, u don't need to pay extra, i'm just saying... they might find a way to jip u off