If you wanna work with AJAX controls you should include the follwing

include the follwing in of web.config file
<httpHandlers>


" validate="false"
type="System.Web.Script.Services.ScriptHandlerFactory,
System.Web.Extensions, Version=1.0.61025.0,
Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>

"
validate="false"
type="System.Web.Script.Services.ScriptHandlerFactory,
System.Web.Extensions, Version=1.0.61025.0,
Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>

"
type="System.Web.Handlers.ScriptResourceHandler,
System.Web.Extensions, Version=1.0.61025.0,
Culture=neutral, PublicKeyToken=31bf3856ad364e35"
validate="false"/>
httpHandlers>