Ajax functionality for ASP.NET can be added by downloading and installing Ajax Toolkit. It is in the form a a DLL that needs to be added to every project, where you intend to use it. It is supplied by Microsoft but rather mainted as open source project where everyone can help, including Microsoft. As a result, you can also download the code and compile it yourself.
Note that there are different version of AjaxToolKit which goes with different version of .NET. Visual Studio 2008 supports only upto .NET 3.5 while Visual Studio 2010 supports the latest and greatest version. Visual Studio 2011 is also in beta and currently not released yet.
To install go CodePlex site and select the correction version that you need. Here is the screenshot that I see
Select .NET35.zip if you are using Visual Studio 2008, otherwise the top version.
Next step is to add it to your project. Open your project. Go to toolbox. Click anywhere empty and and select Add Tab
Give it a name “Ajax Controls” or anything you would like.
Last Unzip the Ajaxtoolkit.zip that you downloaded. Preferrebly unzip into the bin folder of your project. After unzip, drage AjaxControlToolkit.dll and drop it onto the new tab you just created. All of the controls that exist in the toolbox will be available in your application.