I have ntl broadband from a set top box, with 2 desktops and a laptop networked through a switched hub. As far as I'm aware you can't simply switch the usb onto the laptop since you register and initialise a computer to the broadband connection. You would have to do this each time you swap over. A chore to say the least! If the desktop has a network card all you need is a crossover lead to connect the two.
If you don't have said network card right now you could consider doing the whole thing using wireless networking.
Good luck ;-)