When my boss, goes to customer sites, he can not connect to the internet. He has Vista, ultimate I believe. His internet connection does not have a proxy server set up. He has tried, right clicking the network connection to diagnose and repair it, but that does not work. Any ideas on what the issue could be?

Only customer sites but still can access internet?

Even if using Firefox, check this settting
-Open IE, tools, options, connections tab, LAN Settings at bottom
-Make sure nothing is checked there and Apply

If anything was checked and now is not, check connection again to see if can connect