The RadWindow lets you create Child and Dialogs windows, which can have their appearance and content customized. Attached you can find a sample I made for you.
For more information you could refer to you ronline documentation and examples - http://www.telerik.com/help/silverlight/radwindow-overview.html and http://demos.telerik.com/silverlight/#Window/FirstLook
I hope this helps. Please do not hesitate to contact us if you require any further information.
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