Everytime I would add some sort of 'add-in' to VisualStudio 2008 and then would try to get the properties of a project I would get the error, "Impossible to open Project Properties - COM object that has been separated from its underlying RCW cannot be used". Doing a little Goooogling reveals this page, and this where they basically explain that removing Resharper and the, "Microsoft Source Analysis Tool for C#" fixes the problem. For me I removed FXCop and installed the latest, StyleCop. FYI: Generic info re: StyleCop here.