Here are a couple things to try.

1. Make sure your router is pulling an IP Address for your Xbox. Go to your xbox dashboard and go to Network Settings > Test Connection. If the IP Address comes up ad 0.0.0.0, that’s where the problem is. Unplug your router for 30 seconds then plug it back in. If it still doesnt work, depending on your type of router, there might be a switch on the back to Reassign IP Addresses. Try that and I hope this works

I would recommendd if you have wireless broadband that you invest in the WIRELESS adaptor. This enables you to play wirelesly over Xbox LIVE. This can be costly depending on your region but it’s a great investment and easy to set up. It usually asks for a WEP code when installing, just like Nintendo WFC for Wii & DS.

i had the same problem, he ethernet lead worked but wouldnt with the 360 so i bought a new ethernet cable and he presto it worked.

maybe your internet conection is good

your internet connection might be to slow.

Check here to see if it is.

mmmmninja is wrong, wireless fks up your connection, you need to call microsoft at
(800) 4MY-XBOX

either buy a new ethernet cable or buy a wireless network adapter

Well, if the cable still works then you need to have someone look at that 360. I haven’t heard of this problem before, but it seems that the problem would be the system itself.