hi and
Simon.
the router should be acting as a DHCP server - and I would expect the IP address of the clients to be in the 192.168.x.y region.
have you tried disabling XP firewall, jst to be sure that this is not getting in the way of the connection?
the pc with the 81.x.y.z address seems wrong to me - I would not expect that address from any router, unless you have changed the settings.
on the laptop, you say you can see the router from a browser - what address are you using.
also, when you connect to the router - does it show a WAN address (this should be the external address as assigned by NTL)?
can you confirm that the router is lugged into the Cable modem, and the PC is plugged into the router (not the CM).
Cheers
Zovat