You are better off using the Cable Modem for all your internet use. Cable modems can do broadband faster than the STB (assuming you do have the more recent modem versions) of up to 10Meg services, whereas the Samsung STBs can only do up to 5 meg with the Pace STB less than that.
You could ask ntl to transfer the broadband to the STB, instead of the modem, so you wired to a router (with wireless) next to the STB which then allowed wire to the PCs upstairs and wireless for the lappy. But wha't the point if the broadband speed might be less
Or are you saying the wireless connection from downstairs isn't good enough?