a switch won't do any routing, so unless your modem has routing capabilities in it, or you can run your xbox through your pc and use internet connection sharing, i'm afraid you're not of luck.
you need a router, or have your pc do the routing, but for that you need to two lan ports on the pc.
as it stands, your packets have no name tags and therefor no one is directing them were to go.
i suppose you could run both, but not connected at the same time.
i'd check your modem documentation to see if you can make it do the routing, then your switch should work fine. just make sure you're plug the uplink port on the switch into the modem.