Workgroup Messaging?

Displaying my ignorance asks: "We have a small workgroup running on Windows XP Professional; we do not have a domain server. We use Peachtree accounting software which is supposedly multi-user. Unfortunately, multi-user does not translate into allowing two people to be in the same module (i.e., accounts receivable) at the same time. Because the users are in different buildings they often crash Peachtree because they don't know that someone else is already in that module. These crashes result in the loss of data since the last backup. I am therefore trying to locate software, compatible with Windows XP, which would allow a user to create pop-up messages for display on the remote screens. It needs to be a pop-up, not just a flag in the system tray; ideally it would be a flashing neon sign which fills most of the screen [grin]. Because we do not have SIP (Session Initiation Protocol) Messenger appears not to be a solution. Any suggestions?"