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Here's an article on how to turn this service off. If the service is off on your machines you can use these directions to turn it back on.
Disabling Messenger Service in Windows XP [microsoft.com]
Windows Messenger looks like a better solution. It's similar to the Instant Messenger (IM) services like MSN Messenger if you needed the images. You would need some further infrastructure in your network like Live Communications Server 2005, Microsoft Office Communicator 2005 or Exchange 2000 Server Instant Messenger Service.