There are some crucial DLLs for Visual Studio have been removed.
If you remove Extensibility.dll from your GAC temporarily you can reproduce it.
All you have to do is run this command line if you have this problem.
"c:\Program Files\Microsoft Visual Studio .NET 2003\SDK\v1.1\Bin\gacutil.exe" /i "c:\Program Files\Microsoft Visual Studio .NET 2003\Common7\IDE\PublicAssemblies\extensibility.dll"
or you can close Outlook and Visual Studio, and start the install process again.