Ummm.. I just bought Norton Internet security 2005 for my new PC (hooked up to a wireless connection) and when I enable the Norton thing, and go to use the internet, it gives me the "this page cannot be displayed error". When i turn it off, the internet works fine.....
In norton, under privacy, or something like that, there will be a port blocking thingy, make sure port 80 and 81 are ALLOWED. also, disable any other firewall or antivrus software you have running.
Also make sure that the IE/Firefox are under allowed programs. The firewall is blocking it. That's why, when you turn of the Norton Security 2005, the problem goes away.