Question for all you Linux and Network Security people. Could you please recommend a replacement for MSN IM?

Our office manager is having concerns over desktop security and wants to close out the big four, MSN, Yahoo, AOL, and ICQ. One of the other desktop technicians recommended Jabber, but I'm not familiar with it. What would you recommend for a very large company that covers two continents? It will have to be completely self contained and never have to leave the firewall except for updates. The client will have to run in a 99% Win XP Pro and 1% Vista environment. We don't have any objection setting up a Linux server. Management is unwilling to spend anything on it at this time, so freeware is best if possible.