New To ASP.NET AJAX

[shyam_oec]

Hi,
I have installed and configured ajax toolkit assemblies into my VS 2008, and from its documentation tutorial i have tested the following:

Adding a Control to a Web Forms Page from the Toolbox
To use the controls, you can drag them from the Toolbox to a page or you can use the Visual Studio Add Extender wizard. For example, to use an ASP.NET Ajax Calendar control in a Web Forms page, follow these steps:
1) Open a Web Forms page and switch to Design view.
2) From the Standard tab of the Toolbox, drag a TextBox control to the page.
3) From the ASP.NET Ajax Library tab of the Toolbox, drag a ToolkitScriptManager control onto the page. When you do, the System.Web.Ajax.dll and AjaxControlToolkit.dll assemblies are added to the Web application’s Bin folder automatically.
4) Right-click the TextBox control and then click Add Extender. Selecting this option causes the Extender Wizard to appear.
5) Select the CalendarExtender from the wizard and then click OK.
6) Run the page. When you move focus to the TextBox control, a Calendar control appears.

But when running the above code, it is giving following error:

Could not load type 'System.Web.UI.ScriptReferenceBase' from assembly 'System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35'.
.
Any Help in this regard?
Thanks.