This is one Jabber client for .NET you can choose to use:
http://code.google.com/p/jabber-net/[
^].
This is a nice walk-through article on creation of the Jabber client application:
http://aspalliance.com/634_Create_Your_Own_Jabber_Client_With_IPWorks[
^].
Unfortunately, it's written in VB.NET, but you should understand it.
This CodeProject article could also be useful:
http://www.codeproject.com/Articles/21267/Creating-a-Jabber-Client-using-the-agsXMPP-Library[
^].
You might demand to have a complete manual which includes ASP.NET-specific detail of the implementation. I would not advice such approach. Better take one step at a time: first, learn how a Jabber client can be developed in principle, and later think how to use it in a ASP.NET application, where you will face some critical limitations. Otherwise, try to find suitable material by yourself. Why not, after all? Here:
http://bit.ly/10lPy7y[
^].
Good luck,
—SA