When you have cable broadband it has to be enabled by NTL before it will work, if you were to plug your PC into your box it would "talk" to your box and the box would then send a signal back to NTL (or whoever your service provider is) to see if it can allow broadband access. NTL's computer system would then deny the request and the box could not "marry" itself to your PC.
I dont honesty know if the box could be moded to get round this but it hasn't been done so far.
There is no need to purchase a broadband modem for cable as the set top box is your modem, you simply need a network card or a USB connection to connect it to your PC.