You could use the LeftOffset property to move the window away from the Center of the application. You could also not use WindowStartupLocation with value CenterOwner or CenterScreen - this way the window will not center.
If you choose to not use the RadWindow control you may use the WPF class MessageBox and its Show method instead. You might refer to the following example for more information: http://msdn.microsoft.com/en-us/library/aa358525.aspx.
Hope this helps!
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