Look at the router, and at the DNS that is set on that, then configure the XBOX to be set to that DNS address.
It should be the DNS address of your ISP. DNS is your Domain Name Server which translates
www.whatever.com into ip addresses.
Welcome to the Forum by the way!
Also : Have a look here...
http://support.microsoft.com/kb/908884
Note: This actually says set your DNS to be the IP address of the router, so i may be wrong in my assumption above. Either way try what Microsoft says first - then the forum goons.