As above...
I also have NTL, and everytime I want to switch it between my pc and ps2, I have to reboot the modem so that it can pick up the new mac address from the device. If I try switching the cables without a modem reboot, the internet does no work. But after a 30 second reboot it all works fine.
Check the lights on the modem. The 2 lights on the right should be solid. And the ENET light should be on. The other lights in the middle will go on and off when data is being sent/received.
When you connect via ethernet to your modem, make sure the little network connection icon in the task bar next to the time, says "connected". Hover your mouse over the icon. It should be an icon that looks like 2 computers. If it looks like 1 computer with a X, then it means its not connected to your modem.
Make sure your tcp/ip settings in the network connection on your pc is set for obtain IP automatically.
You could also try renewing your ip address, but this will only work if you have a connection to the modem ok. Go to the command prompt and type
ipconfig /renew
And see what it comes back with.