Hi Perry,
You can choose the default save document type in
Tools=>Options=>Save
The .MHT file is a 'single file' web page, and retains
the graphics as encoded content rather than have an HTM
page that links to outside graphics. You might consider
it similar to a .DOC, .ZIP, .PDF file as far as how things
are structured (i.e. one file). Both Word and IE have trouble
reopening large .MHT files at times.
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As an instructor we have used Word to show students
simple web design. The newest version 2003 has a default
setting for saving as a web page to .mht/.mhtl. Can this
be defaulted to .htm/.html when saving as a web page?
The instructors and students are not familiar with .mht
or.mhtl. In fact, we can't find anyone in school that
ever heard of this.
Thanks for you assistance. >>

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Let us know if this helped you,
Bob Buckland ?:-)
MS Office System Products MVP
*Courtesy is not expensive and can pay big dividends*
Office 2003 Editions explained
http://www.microsoft.com/uk/office/editions.mspx