Well, you can change the Xbox IP in the system blade. Look at the IP's on your PC's, which I am assuming are assigned by the router via DHCP. Then if your IP's on your PCs are something like 192.168.1.100 and 192.168.1.101 or like that, then set your Xbox to 192.168.1.120 . If your router uses some other IP scheme like 192.168.0.100 then of course set your Xbox like that.
Also. you would get more help in the right forum. This is Xbox 360 hacking forum. Mods could you move it please?