It took be a while before I realise that while Office 2003 and Office 2007 can be installed side by side, and that it doesn't really matter which version is installed first, it is an entirely different story when it comes to developing Office solutions using VSTO on a development environment with both version installed. In a nutshell, it is not supported and highly unrecommended to have both versions installed when developing using VSTO. The main reason is due to the 2007 Primary Interop Assemblies ......