Hi, I'd suggest opening the Network icon in the Control Panel and removing everything inside it then reboot the machine, let Windows reinstall anything it needs re-installing and start the broadband install from scratch.
Did you have a dial-up set up previously? If so the software from BT may be having problems re-configuring it hence my suggestion above.
But this assumes that the BT software is able to restore the correct network settings automatically which I'd guess it should be able to do.
hth