Emacs; how to override built-in xml-mode?

I just got a recent emacs going on Mac OS X (had to build it, no big 
deal), 21.3.50.1, and these days it comes with an SGML-mode and an 
HTML-mode and hey-hey, an XML-mode.  This is cool, but I've got my own 
XML-mode (which when I wrote it in, dig this, 1989, was called 
structext-mode) and of course it has a function called 'xml-mode you 
run.

Anyhow, I'm having the dickens of a time overriding Emacs' auto-mode 
machinery and convincing it I really want to use mine instead of the 
built-in one.  I thought I knew how this stuff worked but it's been a 
few years... anyone got a pointer to a HOWTO?  -Tim