I ran the vs_piaredist.exe, dropped Microsoft.mshtml.dll into the program directory but I still get the same error.
Do I need to make the Live Support Operator client a fully trusted caller?
Note All post-Beta 2 releases of the .NET Framework SDK now include a primary interop assembly for Mshtml.dll. However, any organization that distributes the .NET Framework through the .NET Framework Redistributable must not depend on the existence of this interop assembly on clients. The assembly, named Microsoft.Mshtml.dll, can be found in the following location
%INSTALL_DIR%\Microsoft.NET\Primary Interop
where %INSTALL_DIR% is the installation directory chosen for the .NET Framework. You can reference this from your projects and deploy your application on any system on which the .NET Framework SDK is pre-installed.
• Because this is an unmanaged COM component that allows only full-trusted callers, you will need Full Trust code access permissions to run the control, as discussed in the "How to Run the Sample" section of this article.
http://support.microsoft.com/?kbid=311295