DOCBOOK-APPS: Newbie: Which validating parser for XML DocBook?

[Piet Seiden <ps@db.dk>]

I'm starting out with XML, trying to convert existing documents using the
DocBook XML DTD (4.1.2). As editor I have tried using XML Spy 4.0 but have
not been able to validate anything, not even simple sample documents. Before
this drives me insane, I would like to inquire here, if there is a
validating XML parser known to work well with the DocBook XML DTD. The
platform should be either Windows, Mac OS X, FreeBSD or Linux.
Thanks in advance
Piet.