ASP. NET AJAX makes it easily to take advantage of AJAX techniques and enables you to create ASP.NET pages, and enables you to take full advantage of the capabilities of the browser to deliver richer Web experiences that work on any modern browser.
how to upgrade an existing asp.net 2.0 web app?
You add ASP.NET AJAX functionality to the application by adding a reference to the ASP.NET AJAX assembly,
I explain how to use ajax using XMLHttpRequest object in details in an article in codeproject.
Ttake your time reading the article and waiting for your comments, questions and queries .