Ok lets run through the setup
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