Pc Intercom will automatically configure the builtin Windows firewall as part of installation. If you wish to do this step yourself, see the following article:
Microsoft.com - Allow a program to communicate through Windows Firewall.
Pc Intercom requires the firewall to allow incoming UDP port 1550 so it can receive Intercom messages from other computers.
If you have a third party firewall such as Norton or Symantec, follow their instructions for allowing access: