If you want to see xhtml as xml just save the following to a reg file
and run it
unfortunately as I understand the problem he wants to save from an
attachment and then run the xhtml file, using a .html extension as a
html file, without IE changing the file. IE was changing the file so
that it was no longer valid against the xhtml dtd which it was before
thus partially negating his ability to programmatically work with the
file as if it were xml at the same time as being able to view it as
html.
One could of course add some commands to the right click menu for html
files, under Open - browse, as xml, etc.
XSL-List info and archive: http://www.mulberrytech.com/xsl/xsl-list