ASP.NET WEB Forms

I have a Webform application that I do not want to rewrite. It works perfectly for the application it is developed for. I have a situation where I perform a lot of code behind. I need a very slick method of when something takes place in the code behind I can send and alert to the user in the web browser. All these message would wait until the code behind is finished. Based on results from the code behind I need to change the message and notify the user. I want this to be easy and intuitive.