I have managed to kill a network card before now - but only because I found I'd wired a lead up wrongly
the point is that it is possible to do and any electronic component can die.
The ntl guy may have suggested this already. Unplug the ethernet / USB cables. Boot into safe mode. then into hardware device manager (windows contorl panel > system) and delete the network card. Now reboot the computer which should prompt a message ater the logon screen (or when the desktop appears) of new device found, and reload the network driver - try to make sure you have the current one (download that now from the internet).
Power off the modem (or STB) and the PC, connect the ethernet cables, reboot the modem/STB and then the PC.
Network cards are cheap if it has died, but if you don't have a spare expansion port
