Never heard of it before. Searched, found the answer. The version in Win2k can only be used to monitor packets sent to the machine it's installed on, it can't listen to overall network traffic. SMS Server includes a full-featured version.
There are other packet sniffing programs available. You should be able to log the packets and filter them based on the ports used. Remember though that it will only work if you're connected to a hub, not a switch. And if the chat program uses anything other than plain text, you won't be able to read it (as far as I know, all the common programs just use plain-text HTML code or their own markup language in plain text).
Presumably you aren't doing this in a situation where you don't have the right to monitor someone else's communications, such as a roommate, boss, coworker, sibling, et cetera.