NEED good information on alternative for dyn:evaluate() for MSXML

I'm sure this question is probably an FAQ, but I've been digging
around the web for a couple of days trying to find a work around for
converting a string to XPath for MSXML4 and haven't been able to find
the right place.
The way I understand it is you have to write an extension, but I'm not
clear on how to do that.  Plus, I have a feeling that someone else has
invented that wheel.
I'm not apposed to switching XSL parsers, but I if I do it needs work with ASP.
Thanks,
~Steve