Your NTL cable feed would connect to your modem. The modem would then be connected to your router. Your router would also have a connection to each PC, assuming you buy a hardwired router.
The router acts as a DHCP server. When you install your router you will access it's settings through an IP address entered into Internet Explorer, this will access the routers internal software/settings & allow you to set up the router. From this point you should clone the MAC address of the NIC that was connected directly to your modem before. You should now be able to connect to the internet on both your PC's (in theory!)
Some routers have a built in hardware firewall, but you should still install software firewalls on each PC that connects to the internet. There are plenty of free firewalls around that will do the job very well, I use Sygate standard.
The router will act as a DHCP server on your network, I say network because you will be able to connect to each computer through the router if set up correctly. DHCP is, I think Domain Host Control Protocol, but don't ask me what it means, networking gives me headaches!! Somebody else on here will be able to explain it better I am sure.