Today, when I started visual studio and wanted to Add new Item to an existing project / solution, I found that the Template for Silverlight Child Window was missing from my visual studio 2010 templates.
Thanks to Eric Hammersley who gave the perfect solution. To add missing templates you need to run following command (Elevated!) on visual studio command prompt.
Close all instance of Visual Studio. Open visual studio command prompt and type,
Press Enter. Let the process be complete and now open visual studio. You will get all missing templates under Visual Studio installed templates.