The problem might be in the BT router. I do believe BT host instructions how to port forward the device.

oh .. found other instructions. (1) Setting up a Static ip address for Windows XP (this must be done before step 2.)

(2) Port Forwarding for the BT Home Hub

Port unstable: LW is behind a NAT or firewall and has tried to discover its external address and port, but different peers have reported different ports, which may indicate that the NAT/firewall device isn't suitable for accepting incoming connections.

You should be aware, like most ISP's in UK nowadays, BT shapes p2p bandwidth during various times of day using a special filter.