Screen Popup allows you to send an instant global popup message to all PC's on the internal network that you install the included agent on. Unlike a lot of similar applications, this does not use the Windows Messenger service so will work on Windows XP and 7.
It will also display the popup message on all PC's no matter which subnet they are on.

Create a shared folder on a server or PC that's always available, with full permissions to all staff to read/write to. For example, on a server called 'servera', create a shared folder called 'ScreenPopup_Relay'.

Staff should then be able to browse to the share via \\servera\screenpopup_relay

This share will be used as the messages repository.

The license of this software is Freeware, you can free download and free use this lan messenger software.