Error when I choose an item by it's image

I am setting up an online store for our business, in a 'store' folder off of the mail root.
http://www.creativeventures.biz/store/

When I go to choose an item by clicking on an image from the main page, I get the following error (Log)


Exception of type 'System.Web.HttpUnhandledException' was thrown.  
Exception:  System.Web.HttpUnhandledException: Exception of type 'System.Web.HttpUnhandledException' was thrown. ---> System.TypeLoadException: Could not load type 'System.Web.UI.ScriptReferenceBase' from assembly 'System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35'. at AjaxControlToolkit.ToolkitScriptManager.OnResolveScriptReference(ScriptReferenceEventArgs e) at System.Web.UI.ScriptManager.RegisterScripts() at System.Web.UI.ScriptManager.OnPagePreRenderComplete(Object sender, EventArgs e) at System.Web.UI.Page.OnPreRenderComplete(EventArgs e) at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) --- End of inner exception stack trace --- at System.Web.UI.Page.HandleError(Exception e) at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) at System.Web.UI.Page.ProcessRequest(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) at System.Web.UI.Page.ProcessRequest() at System.Web.UI.Page.ProcessRequestWithNoAssert(HttpContext context) at System.Web.UI.Page.ProcessRequest(HttpContext context) at ASP.product_aspx.ProcessRequest(HttpContext context) in c:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\Temporary ASP.NET Files\store\1bb68126\552afb3d\App_Web_4hqtu8lz.18.cs:line 0 at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)  

Any help would be appreciated...!

Please ensure that you have .NET 3.5 SP1 installed

THANKS Andrei,
I moved the site over to a server that was running 3.5 v1 and it cleared up the error message.

I appeciate your help - tremendously!