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As danielf indicates above a router is going to be your simplest option, if Internet Connection Sharing is not convenient, and you build lots of these things (a question springs to mind as to how much this all becomes business use, rather than residential but that's a different matter).
You can connect direct to the STB, but you will need to disconnect your other PC, and reboot (power off and on) the STB. The new PC wil then need to register with the ntl server. Usually entering any web address will by default take you to the registration page. If not type "start:ntl" You'll need to know your customer ID details.
You can only register a few PCs via the STB (3 or 5 not sure). After that you have to start deleting them. As you say it works by mac addys.
NB if you use a router, again that has to be registered with ntl (or clone the mac addy of your existing PC), but at least you only have to do that once.