lightweight Chat client/server?

[Depo Catcher]

I have a combination of Spark (windows client) and Open Fire (FreeBSD 
server, actually Java) for my lan.
We've used this setup for years, but the OpenFire server takes up ~500 + 
MB.

Anyways, we were looking for something a bit smaller.
We just need to send text messages to LAN users (less than 6) and 
supports a nice windows client.
We're not suppose to use any external services (yahoo messenger, aol, etc)