First off, you mention an ethernet you mean the router or do you have a hub connected to your router, and the XBox connected to the hub?

1. Make sure you are not using a crossover cable.
2. Reboot router after connecting XBox.
3. Try a different cable.
4. DHCP on router enabled?
5. All items on the XBox's network menu set to "Automatic"?
6. Your router may not be compatible with the XBox. Try a different router.