If all of the computers are set to get their IP addresses automatically, they should each get an address in the 192.168.1.x range. If one PC is getting an address automatically which is not in this range, something is not connected correctly.
You don't say which type of broadband service you use but, if it is Cable (NTL/Telewest), the router's WAN port must be connected to the Cable Modem and the PCs connected to a LAN port (or "wirelessly").
I could be off the mark but, reading between the lines, it sounds as though you have Cable broadband and have connected the modem to a LAN port instead of the WAN port. The result of this is that one of the PCs will get the IP address allocated by the ISP and have internet access, the other(s) might get a 192.168.1.x address from the router, but won't get online.