If you installed latest version of Silverlight (Silverlight 4), it may break the your current silverlight development environment. After in the Silverlight 4 installation, if you try to debug any Silverlight 3 project, you will get a message from Visual Studio 2008, saying “Unable to start debugging. The Silverlight managed debugging package isn’t installed.”
To fix this error, install the Silverlight Developer Run time. You can get it from here : http://go.microsoft.com/fwlink/?LinkID=188039.