Review the following online help article: General Troubleshooting
. Make sure that you have the Telerik.Web.UI.WebResource handler in your web.config file.
Also please make sure you are using the right .NET build of the Telerik.Web.UI.dll assembly (.NET 2.0, 3.5 or 4.0 depending on your target framework). The .net version is the last number of the assembly version:
the Telerik team
Consider using RadControls for ASP.NET AJAX (built on top of the ASP.NET AJAX framework) as a replacement for the Telerik ASP.NET Classic controls, See the product support lifecycle here.