Hello Kevin,
When you call the __doPostBack function you actually postback the page as the function's name says. Would you please explain what exactly you want to achieve by calling __doPostBack when closing the RadWindow? Once I have a better understanding on what you are trying to achieve I will do my best to help.
For the time being, I recommend to examine the following article which explaines how the __doPostBack function works:
http://geekswithblogs.net/mnf/archive/2005/11/04/59081.aspx
Kind regards,
Svetlina
the Telerik team