OK - when you reinstalled the main PC, did you reinstall the Telewest USB network drivers?
Robin Walker's page points to the Motorola drivers page
here (expand discontinued products at bottom!)
Once you got the drivers loaded, turn off the main PC & modem. Plug the USB from CM to PC, unplug all Lan cables. Turn on the CM and wait 30 sec. Turn on the PC and see whether it will connect to the internet/modem.
HTH