After the Silverlight runtime update to 4.0.60531.0, opening a XAML file in design view was crashing visual studio. There was also a problem in opening the Toolbox, or any designer that loads the toolbox, with Visual Studio crashing when "Loading toolbox content from package 'Microsoft.VisualStudio.IDE.ToolboxControlsInstaller.ToolboxInstallerPackage' {2C298B35-07DA-45F1-96A3-BE55D91C8D7A}" When I found the solution, it appears this probably only affects systems where 'System.Windows.dll' has been previously registered in the GAC. The solution, is to remove System.Windows from your GAC using the following command from the Visual Studio Command Prompt: gacutil -u System.Windows