Which version of the RadControls for ASP.NET AJAX are you using? Please note that we support IE9 since Q1 2011 and older versions have problems with it, one of the most notable is with the RadWindow, since IE9 requires that iframes are handled differently. Please examine this forum thread
for more information on the matter and a possible workaround if you cannot upgrade to the latest version for the time being (which I strongly advise that you do as soon as you can).
the Telerik team
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