xxx is not set up to establish a connection on port "WWW (HTTP)"

It seems many people have had this problem, but I have not yet found an
answer as to how to fix it. I am trying to connect wirelessly to my
router/modem (Zyxel Prestige 600), and thence to the internet. While my
system (running Vista Home Premium, upgrade from XP) connects to the router
fine, and even suggests I am connected to the internet ("Access: Local and
Internet"), when I run Explorer or Firefox, I cannot access any sites.
Following the windows "diagnostic", I get the following error message:

'"www.google.com" is not set up to establish a connection on port "World
Wide Web Service (HTTP)" with this computer'

Although I cannot access any web sites, curiously the system was able to
access and install Vista updates.

If anyone can help me fix this I would be most grateful (although please
bear in mind that I am a simpleton, so you will need to speak very slowly).

Many thanks,

Emma