When you use a router, the router can "clone" the MAC address of the computer currently connected to the broadband modem.
This means that, as far as Telewest are concerned, it is still the same machine connected to the modem and not the router.
Any machine which you now connect to the router can use the internet without having to register the MAC address with Telewest.
Normally routers are very simple to set up. Follow the instructions that come with the router and you should have no problems. It should just be a case of plugging the cable modem into the router (via network cable - not USB) and then connect each computer to the router via a normal network cable (i.e., not a crossover). Run the setup program CD (which should come with the router) and that will sort everything out for you.