First, a security warning: MSN IM traffic travels unencrypted and is stored on Microsoft servers. If you're doing company business that way, I'd suggest you explain to the boss why that's a bad idea.
There was a recent TR debate on whether IM even belongs in the workplace. The only advantage of IM that I can think of is the chat and voice features can replace long distance telephone calls. But again, you should understand that it's easier to intercept than a phone call.
MS isn't very forthcoming in their knowledge base as to how IM works (as far as ports and protocols). If you sniff all traffic going to and from messenger.msn.com you should get the IM traffic. Depending on the features of your sniffer, it make take a lot of effort to sort the legitimate traffic (if any) from the abuse.
If you monitor in this way, you must check with the company lawyers on the company's monitoring policy. You have to know how deep you're allowed to dig, and what you can do with the information you find. This came up in the news recently, because some judges declared monitoring illegal when they found out their own surfing was being monitored (even though the government has monitored for over a decade, to protect from abuse of taxpayer dollars).
My suggestion is to use the security argument to get the big boss to outlaw IM in your company. Then, you can block it at the border router or firewall.
Good fortune,
Don
