I'm just quoting this but it sounds good to me:
"The best way to create MHTML documents is by utilizing Internet Explorer itself. If you want to save a webpage, go to that webpage,
then in IE, click File>Save As. In the field "Save as Type", locate "Web Archive, single file (*.mht)", then click Save.
That should be it. This answer just applies to webpages, or already saved HTML saved
To edit the MHTML documents, you can open the file in IE, save it in "Complete web page format (HTML + Images)
Then Open the HTML file in Notepad, edit it, open in IE and save it back as MHTML."