From what i can gather, Windows Messenger is more to do with communicating over networks. If you have XP Service Pack 2 installed then Windows Messenger is disabled by default.
.Net passports only work with Microsoft stuff.
You cannot cross platform chat e.g if you've got MSN Messenger and a friend has got Yahoo Messenger you cannot chat together. You both need the same chat client.
I used to use ICQ and MSN Messenger on my old Win ME comp.
Hope this all makes sense.