In your browser go to the Tools menu and select Options/Internet Options (depends on which browser as to its exact name). Within that, the first tab is titled General and the top part of that section is for setting the home page to whatever you want it to be.
If you are getting a BTfon home page it means you are connecting to a BTfon wireless access point rather than the wireless that provides you with your normal Internet.
Remove BTfon from your list of preferred wireless networks.
in vista/windows7 it's Start button -> Control Panel -> Network and Internet -> Network and Sharing Center -> Manage wireless networks (in left pane) -> highlight the BTfon network and select remove