Duplicate thread has been removed.
Assuming you have purchased a cable compatible router (not one with an ADSL modem):
Power everything (modem, router, PC, Xbox off), and disconnect any USB or ethernet cables between them.
connect ethernet modem to router WAN (internet)
connect ethernet router LAN (network) to PC
connect ethernet router LAN (network) to Xbox.
Fire up modem & let settle
Fire up router & let settle
Fire up PC and let settle
Fire up Xbox and let settle
That should be it.