just as something to try, change the xbox's ip address to 192.168.101.9, make sure you hit save and exit, then restart your xbox with it hooked up to your computer. i had a similar problem (but i was using a router) and i changed one number from the third section of the ip address and it has worked ever scince, and if that doesnt work, its easy to change back
oh, and make sure your xbox is set to use a static ip