sounds exactly like something my boss would ask me to do. Or at least some other department would ask my boss to ask me to do.

I'm pretty sure there are IM capable forum apps. I run Ikonboard and it has IM capabilities but not really realtime. More like an ICQ thing where you only get notified of the message when you refresh or click to another section of the forum.

As for NT authentication, I think they might be asking a little too much there. Especially with a web-based (php, perl, cgi, etc) forum. I could be wrong, but I think it's even hard to find any app out there that uses NT auth that either not by Microsoft or from one of the larger popular companies that probably work directly with M$.

I could be wrong there, but I assume it's near impossible to code your own app using NT authentication and almost as hard to find some ready-made app/plugin to integrate with your app.