I believe that the How to Use RadWindow with AJAX help article
will answer your questions on the way to use AJAX with the RadWindow. Let me know if you have any further questions in this regard after examining it. You may also find useful the RadAjaxLoadingPanel
control from our suite or the UpdateProgress
control from the standard AJAX suite.
the Telerik team
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to their blog feed