I'm going to go out on a limb here (Score 2, Interesting)

and suggest a (gasp!) commercial solution. Not free, but supported: Sametime. Commercially supported by IBM/Lotus, fully secure, with a built-in web conferencing system, and works on your Windows 2003 server. Can be completely stand-alone, or you can have it authenticate to your company's LDAP directory. The nice thing is you buy only the number of clients you need, with no need to purchase server software. Clients are $47.59/user, and allows you to use the stand-alone Sametime Connect client (Windows, Mac or a Trillian Pro plugin), the browser-based chat client, or connect via your Blackberry if you're already using their Enterprise server 4.1 or later.