Hi,
The version that you use - RadWidnow for ASP.NET does not support IE8. We provide full support for that version of IE in the Q1 SP1 2009 (ASP.NET AJAX) version of the control. I strongly recommend to consider upgrading, but if you insist on using RadWIndow for ASP.NET, you could try the following workaround - just place the script on the page after RadWindowManager's declaration:
Regards,
Georgi Tunev
the Telerik team