If any firewalls are installed on any of the PCs, uninstall them.
This includes Zone Alarm, Sygate, Kerio, Norton Internet Security, Zero Knowledge Freedom, Cisco VPN Client, F-Secure, McAfee Firewall, Trend Micro, Comodo, PC Tools Firewall Plus etc.
In many cases, disabling the firewall is not enough; you have to actually uninstall it.
Do not install or re-install the firewall until after you have the network functioning properly
Web Browser Settings
Internet Explorer 4.0 or Higher
Start IE web browser. Select-Tools-Internet Options-Connection Tab
In the “Connection” screen, there are three selections:
“Never dial a connection”,
“Dial whenever a network connection is not present”
“Always dial my default connection”.
If you can make a selection, select “Never dial a connection”.
If you cannot make a selection, select “LAN Settings”
Make sure there are no check marks next to any of the displayed Options:
“Automatically detect settings”,
“Use automatic configuration script”
“Use a proxy server”.
Click “OK”. Then click “OK” again in the “Internet Options” page.