I recently started getting an issue opening any XAML file in the designer in visual studio. Visual studio would crash immediately. This then started to bleed over to crashing when trying to open the solution. I tried everything included reinstalling VS altogether. Didn’t work. When reapplying the Service Pack it downloads the Silverlight Developer tools from the internet. This version is 4.0.60531 which appears to be the culprit. Colleagues have an older version and have no issue. The error in Windows Event Viewer was fault in module agcore.dll. After a quick google i found this post with others having the same issue. The solution was to unregister System.Windows from the GAC. Presto. Works.