xml and javascript

can someone put me out of misery please...

Having realised that xmlHTTPRequest is great, but HTML has limitations - 
in particular if I want to update a DIV with the result and execute some 
Javascript I can a) make up my own syntax or b) try to use builtin 
Javascript functions for processing XML.

Hmmm... this seems to be browser dependent. Is this correct? I need XPATH.

I have a simple vocabulary to do the work - basically:

<response>
    <update id="xxx"> </update>
    ...
    <execute>...</execute>
    </response>

where updates go into innerHTMLs of the id's and executes are handled by 
eval(); (the application takes care of making sure the updates are valid 
html).

(I know I should probably ask on the Mulberrytech list - but I've never 
had success with that list - have to change all my email settings to use 
it - unlike this friendly list).

I'm looking for generic (ie IE + Firefox at least) solutions - so MXML 
is possible but it seems only available in IE and I can't find any 
simple solution for FF.

All pointers and links accepted with grateful thanks.

Rick