Dee747
You cant replace the cable modem with a router they do subtlely different things, however you should have a RJ45 connector on the modem which you can connect a router to and then connect PC 's by wireless or ethernet cable.
If you have already got the PC on the modem I found the easiest way with NTL is to spoof the mac address of the router to that of the current PC, saves all that broadband medic rubbish the NTL like you to install. If you need to know about how to do that ask here.
I use a linksys wireless router on NTL to about 3 desktops and 3 wirelss PC's ( when all the students are home) and no complaints.
Rickity