I have an apple-based wireless network with an airport extreme base station. My Windows XP-PC laptop connects to the wireless network fine. I have a BigPond broadband permanently-on internet connection which works fine with the apple laptop. But Internet Explorer on the PC laptop will not connect to the internet via the wireless network. How do I configure Internet Explorer to use the wireless connection???

I am a little confused. You say,

My Windows XP-PC laptop connects to the wireless network fine.

but then you say,

Internet Explorer on the PC laptop will not connect to the internet via the wireless network

Are you saying you know your laptop is connecting to your wireless network? If so, how do you know?

If you can see other computers on your local network but not get past the router/broadband modem, then it would seem your wireless connection is set up properly on the laptop side. This could mean your laptop firewall is only allowing local (trusted zone) access, but not Internet zone access. Or it could mean your Air Port is not allowing the laptop to get out.