So when you double click on it, it doesn't open in a browser?
Is the icon for the file a browser icon such as IE or Firefox?
Right click on it and see if you get an option to "open with..."
First I get a window that says "unknown file type". The choices are open or save. If I right click the choices are open, save, or save all.
It opens a blank page with a header that says c:/'documents and settings tstillings sometimes it will have the original email message, but never the attachment.
Although both "eml" and "mhtml" file types are in my list of file types, and both are associated with (open with) I.E., it seems to not recognize them.