Windows Explorer search not seeing Word XML documents?

After using Office 2007 for nearly a year, I've just discovered
something odd: when I run a search in Windows Explorer for a document
I created in Word, saved in the Word 2007 XML format, if I ask W.E. to
search for any documents it won't find the file I'm looking for, but
it will if I tell it to search "All files and folders".

Is it that Windows XP does not recognize Word 2007 XML files as
documents? Is there something I need to change either in Office or in
XP's registry to get the OS to recognize Microsoft's own document
format as a document?

Or am I doing something else wrong?