I posted this Visual Studio error the other day, which shows up when you uninstall a VS add-in that tries to take a few critical dll's with it (which seems to be about half of the add-ins I've tried). Pete Davis explains what to you can do to try to fix this problem, without reinstalling all of Visual Studio.
I should point out one minor difference I noticed. In my copy of Visual Studio 2003, extensibility.dll and VSLangProj.dll were both located in C:\Program Files\Microsoft Visual Studio .NET 2003\Common7\IDE\PublicAssemblies rather than C:\Program Files\Microsoft Visual Studio .NET 2003\Common7\IDE\.