Internal Instant Messaging Client and Server Combo

strongmantim writes: "I work for a medium-large company with a decent-sized internal help desk (25-30 people) that I manage. We're looking for an internal, secure, FOSS (if possible) instant messaging/presence awareness client and server combo. Healthcare is our industry, so transmission of PHI (protected health information) is a sensitive area. Therefore, the server has to be able to log any conversation that occurs. It is preferred that outside protocols (such as AIM, MSN, Yahoo, etc.) are not supported by the client, though if it does support them, it will be one more policy for me to enforce that my techs do not connect to them. All of the computers connecting to it are Windows XP and it should be scalable up to 100 people or so (in case we decide to include our entire office in the roll-out). Hardware and OS for the server are not an issue. Oh, and one more thing: It has to be free. Suggestions?"