Hi &
Your ntl broadband STB or modem can only support one connection from a PC at a time. To support more than one at once you need a router between your STB / modem and computer(s), unless you have one PC acting as a server connected to the modem, with all other PCs connected via the swithc to the server PC.
If you connect everything as is to the modem, via the switch, you would need to reboot the modem each time you changed with PC is in use. As you can only have 3 PCs logged with ntl that places a limit too. The router is the best option.
Have regard to the T&Cs or AUP whcih limits the number of computers (including games consoles) you can have conected to ntl broadband.