the MSN dll is available thru visual studio so you can start there

Coversant offers the SoapBox Framework, which is a full .NET SDK for Instant
Messaging using the IETF approved XMPP standard. The SoapBox Framework
offers full XMPP support, as well as quite a few other features such as
conferencing, security, and configuration options. The Framework also runs
on the .NET Compact Framework, and on Mono.

Information on the Coversant SoapBox framework can be found at:
http://www.coversant.net/portal/DesktopDefault.aspx?tabid=48

There is extensive documentation and samples written in C# and VB.NET.
Samples include a full-features (although 'primitive') client, and specific
samples centered around nearly all key features provided by XMPP.

The anwer to that question really depends on what you're looking to do. Are
you trying to connect to the propietary networks and send your friends
message? Are you tyring to build an infrastrcture centered on IM and
Presence? Are you trying to provide secure, federated Instant Messaging to
an orginization?

The answer, depending on what you're looking to do, varies quite a bit....