How to specify a Processing Instruction?

[ComCity <mikeb@comcity.com>]

I need to create an MSXML document with a specific Processing Instruction
and I'm finding it impossible to do.  First, I tried create a template XML
document on disk that I read in and modify it.

Unfortunately, the XML file opens fine when it says this at the top
<?xml version="1.0"?>

but if I change it to this, which is what I need, it won't open.
<?xml version="1.0" encoding="ISO-8859-1"?>

Why?

I can't even seem to be a way to create a new XML document by scratch (or
one that I can find) where you can create a new processing instruction
beyond one that contains the version only even modify an existing XML
document where you would change the
processing instruction.

There's IXMLDOMProcessingInstruction ....

Very little documentation on it, but it only allow appears to let you set or
change the XML version.  I need to set the encoding as well.

I need to be able to send an XML document and the processing instruction
mus be:  <?xml version="1.0" encoding="ISO-8859-1"?>

Why is this so hard?
Thank You
Michael Barber