.net remoting and object invocation

Hi all, I am a beginer in .net and i hav to implement a chat program [ web application, not standalone] and i have implemented public chat, but in private chat if a user send a mesage to another, i wanna pop up a window in the second user's computer [ like all other chat servers]. How can i invoke a remote object from server [ to pop up a window in client system from web server ]? please advice. Thanks to all. Ali