You can only connect either the USB or the ethernet at any one time - not both at the same time. You only have one IP address & the modem cannot share it.
You need to power cycle the modem in between changing connections.
http://homepage.ntlworld.com/robin.d...b.html#usbswap
http://homepage.ntlworld.com/robin.d...tips/swap.html
THis is an old sticky I wrote ages ago, but it should still be pretty relevant:
http://www.cableforum.co.uk/board/15...onnection.html
Make sure the network settings on the Xbox are on auto for IP & DNS.
Really, it'd be a lot easier with a router. No swapping cables, no rebooting the modem, PC & Xbox can be online at the same time.