Simple internal messaging in ASP.NET C#

I want to make a simple internal messaging using framework 2.0 in my web application. Requirement: 1. Message window with multi-list admin users (online admin users only) and text-box with 200 characters privilege with a send button. 2. Message sent from one user to the online user will be displayed as pop-up on their screen. 3. Last 5 messages only shall be displayed. 4. The messages life will be restricted to session only as they will be temporarily saved, Not in DB. I had never bulit any messaging system so please help me to achieve above task. I do not want to use any third party API. Please reply how to do this.