problems validating xml with msxml parser

I need to validate the xml requests that I receive, but I cannot figure out
how to do it.  Does anybody know how to do this using the MSXML parser?

Here is some background info.  My users post xml requests to my page (via
MSXML's xmlserverhttp).  I process the xml, figure what they want, and send
an xml answer.

The request xml has to be well-formed (of course), but it also must be
valid, i.e., it must conform to the schema that I have defined for xml
requests.  I cannot count on my users to do anything correctly, e.g., I
can't count on them to put in the correct namespaces in their docs.