ASP.net Ajax

ASP.net Ajax is Microsoft's free framework for creating Ajax (Asynchronous Javascript and XML) web applications. At its simplest, it lets ASP developers do this - or add Ajax controls to their existing applications - without leaving their familiar drag-and-drop environment, and without having to understand Javascript or asynchronous communication with the host. According to the O'Reilly Network, it protects developers from "the underlying gibberish that makes it [Ajax] all work".

View State Rendering

The problem in earlier versions is that the viewstate is rendered at the bottom of the page.
If the page is posted back before the entire page is loaded, the viewstate that is being submitted back to the server is incomplete and thus invalid.

This was fixed with .NET 3.5 SP1.

But You can Fix it by below Render function in earlier version of .NET