ASP.NET AJAX Microsoft tutorial
Many people asking about the previous link of ASP.NET AJAX 1.0 documentation that started with http://www.asp.net/ajax/documentation/live which support .NET 2.
Actually, this link has been removed but instead you can visit http://msdn.microsoft.com/en-us/library/ee341002(v=VS.100).aspx which illustrate the version that Supported for .NET 4, 3.5 .
Hope this help.