Develop Office Business Applications using Visual Studio
Aaron wrote a great article about the VSTO runtime components earlier, which explained the Office extensions for .NET Framework 3.5 and Office extensions for .NET Framework 4. He also explained that starting in Microsoft Office 2010, the Visual Studio 2010 Tools for Office runtime is included and does not need to be deployed alongside your Office solution – BUT – (currently) only if you are targeting the .NET Framework 3.5.
If you are targeting the .NET Framework 4, the Visual Studio 2010 Tools for Runtime must be selected in the Prerequisites Dialog Box and installed on end-user computers. This is because the Office extensions for .NET Framework 4 are not included in the Visual Studio 2010 Tools for Office runtime that is included in Microsoft Office 2010 Beta or RTM.
Mary Lee, Programming Writer.