You have Microsoft Office Click-to-Run 2010 installed on the computer.
You right-click an .htm file or an .mhtml file, and then you click Open with.
In this scenario, Microsoft Word is not listed as an application that can open the files.
This issue occurs because Office Click-to-Run 2010 is installed within the virtual application space that is created by the Microsoft Click-to-Run technology. Therefore, Microsoft Word is not listed as installed on the computer.
To open the files, start Microsoft Word 2010, and then you open the .htm file or the .mhtml file.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.