You need to replace the DialupBluetooth file created by the Bluetooth network connection in /etc/ppp/peers/ with BT-WINXP and then edit the /dev/rfcommX line in that so it matches the entry in /etc/bluetooth/rfcomm.conf that refers to your PC. The only thing to watch out for is that every time you change a setting in the Bluetooth network connection, it will reset the file in /etc/ppp/peers.
You add another entry in /etc/bluetooth/rfcomm.conf for rfcomm1 for your PC, so that it does not affect your mobile phone connection.