go back to basics, is the NC showing up in Network Connections, i assume yes, are you useing a (wrong)crossover or (correct)standard straight through rj45 cable.
to check that, hold up both ends of the rj45 connectors together and look to see if all the colours match pin for pin (straight through)or there are 2 colours that are swaped over and hence are X over.
assumeing an NTL 250 CM can you browse to the modems internal web server at
http://192.168.100.1 and log in with root/root ?, if so then your using the right rj45 cable, if not then perhaps you have a crossover/Xcable and thats wrong so you cant get anywere.
assuming straight through cable to your CM:
assuming your running win XP ,start/run/cmd then type
ipconfig what does that show?.
as a temp fix you could always install the USB driver and use the usb cable for now until you sort the rj45, again you dont need to install all the crud from the ntl cd,you only want the usb driver to make an OS see the device as a network card.
if you can get access to the net somehow and have a cd/dvd burner then you might download a linux live cd and burn/boot that to rule out your current installed OS , good for testing your ntl connection on the fly
slax live cd is a good and small self booting cd that wont write to your current OS drives for instance
http://www.slax.org/download.php
http://slax.linux-live.org/forum/