I work at a large company in Natick that has recently decided to disallow instant messaging clients from being used on corporate PC's. They sight a concern that it is an insecure network link to the outside world that could allow hackers, worms, trojans, viruses, etc ... into the otherwise protected network. Whether I agree with this opinion or not I guess is irrelevant as I don't have a strong enough voice to change anything. What does affect my team is the ability to easily communicate with each other on a "live" or instant platform within the company firewall (we have no interest really to communicate outside the firewall yet). I was wondering if anyone could point me in the direction of an Open Source alternative to an instant messaging platform? For starters I'm just looking for software to run on a server (either Linux or Doze) and the associated clients to use on primarily XP or 2000 desktops.

I'm doing some research on Source Forge and other sites to see if I can find something myself, but would prefer a suggestion from someone who's already implemented something like this.