Are you hooking it up directly to cable\dsl modem, router, switch, hub.
If to a DSL\Cable Modem:
Typically modems will lock onto a MAC address so if you had it hooked up to your computer then plug it into your xbox it won't work the Modem looks for that MAC. What you would want to do is turn the modem off then back on to reset it. Don't forget to do the same procedure when you are hooking the PC back up lol.
If to a Router:
Well everything should work LOL
If to a switch\HUB:
Typically you wouldnt ever use this with what your doing since this device doesnt do DHCP ( assign IP address ) and a modem only assign 1 IP address to you.
Well let me kno if you need more help. I do this kinda stuff for a living so I don't mind.