Ok, the problem is that your ISP is filtering MAC addresses. (Best guess.)
Follow these directions:
1. Click Start, click Run, type cmd in the Open box, and then click OK.
2. Type ipconfig/all.
Note If ipconfig/all does not work, you can also type winipcfg.
3. Your computer's MAC Address is listed under Physical Address. Enter this information into the Xbox console.
Note Do not use any address that begins with 44.
4. Turn off the Xbox console, and then turn it back on.
5. On the Connect Status screen, select Settings, and then press the A button.
6. Select Advanced, and then press the A button.
7. Select MAC Address.
8. Enter the information under Physical Address. You do not have to include dashes.
Taken from
http://support.microsoft.com/kb/817748