AFAIK even though it is java based - it has to be hosted on your internal server, which is a good thing as you do not want an external based messaging system (ICQ, yahoo, etc).
So in that case it should be pretty secure (needs the server client, as well as the user client). Basically you shouldn't have anyone sending obscene things as it is hosted on your own server.http://www.avoware.com/intchat/features.shtml
I wouldn't outright dismiss it, it has some good features like keeping the history of the messaging, file sharing (can be disabled), work time counted (not sure how useful that is).http://messenger.softros.com/
I found that as well. A payable version that appears to start at 11$ per client. Doesn't require internet connection, as well doesn't require a server component.
The only thing I found was that it mentions it saves messages - however lacking a server component it appears the messages would be stored locally (not a good thing).
There's also this: Pink Noteshttp://www.alphamedia.net/products/pnp/v40/index.htm
It is a similar install to the one I suggested, comes with 30 day free trial but not sure of the cost. It requires a server component as well as a client component - and messages can be archived.
It seems like a decent alternative and possibly what you might be looking for. I'm sure somebody else could suggest something better.