Have  you  tried using XMLSchemaCache object?
Basicly you can load your schemas into a schema cache before 
loading/validating xml file then you can use these schemas to validate your 
xml file using DOM/SAX.
Here is a link to an example
http://msdn2.microsoft.com/en-us/library/ms753784.aspx

-- 
S. Huseyin Ulger [MSFT]
MSXML Dev


"Jason Tost" wrote:

> At this point I don't know if the concept is even sound according to XML
> convention; I am wondering if there is a way to apply a schema I have
> locally when parsing a document received from an external source.
> Essentially, rather than validating for the purpose of determining
> whether a document is well-formed according to a schema that it declares
> it conforms to, I want to validate that a document generated by
> something else is well-formed according to an offline schema I install
> with my app.  In essence, I want to use XML validation to verify that
> the contents of a data file are valid, however this must be done on
> systems that do not have access to a network connection necessarily, and
>  the installation path, and therefore the location of my xsd file, can
> vary between installations.  I do not see how to, using MSXML6, to apply
> my own schema in the validation of a document; all of the examples I've
> seen have the document calling out the schema that it will validate
> against, which would require the XML document to be hard coded to some
> universally accessible path.
> 
> Thanks all!
>