You would like to settle a little instant internal mail service in your enterprise, in order to get your users to communicate between them without having to move or phone. A software like the old msn messenger, but adapted to the companies, more secure.
It will be better if this mail servise was not on Internet, because the company prefers to not give Internet to its employees, as they would spend their time discussing with private individuals or surf on websites.
What could be the solution ? a freeware one.
With Internet the answer would have been easy, with the use of Skype, or Gmail, Facebook, Whatsapp. But without Internet the answer becomes more complex indeed. The best solution is probably SendYoo. It is free for 3 users only. It is a professional instant messaging software that allows to send SMS and mails. No sever to setup.
Otherwise InstraMessenger (totally free). But that need a sever with PHP-MySQL, you can install it with EasyPHP (virtual server on each individual computer). The authentification through another database such as phpBB, vBulletin, Joomla, Drupal, Typo3, or WordPress is possible.
Another possibility is to use internal Windows components : start > execute > winpopup or winchat. You can test Netmeeting. You can also try realpopup that is similar to these Windows applications.
Another lan software is Prochat. It is dedicated for these interactions between workers among a same company.