Sending a message to users on a LAN

I would like to write an application that given a Windows2000/XP username can send a message to that user. If the user is logged in to 3 machines, then I want to send a message to all 3 machines. I haven't been able to find any .NET classes to accomplish this, I have an idea using the Win32 API, but it seems complicated, and I would rather use some .NET foundation classes if there any. Does any one know how to do this with .NET?