I had been running a beta of Visual Studio 2008 and when I uninstalled the beta and installed the RTM version, I found that I had no syntax highlighting or intellisense when editing config files (web.config, app.config, etc.)  When I tried to change the properties in the IDE, I received An error occurred loading this property page from the tools -> options menu.

 To fix the issue I ran the below command from the Visual Studio 2008 command prompt:

 devenv /setup