Problem with Vista/Office 2007 and WebDAV

We have numerous machines running Vista and Office 2007 that receive extra
password prompts when trying to view a document from an intranet web site
that is in the "Trusted Sites" zone in IE. They will be prompted three times
before receiving the document. Clicking 'cancel' on the authentication dialog
opens the document.

We have tracked the issue to several extra HTTP OPTIONS and PROPFIND
requests that are being issued to the web server as the document is opening.
Some of these requests are to the folder containing the document, others are
to the root of the site. The web site does not support WebDAV, and does not
need to.

Stopping the Web Client service on Vista resolves the issue, but this is not
possible for us since other internal sites do rely on webdav.

How can we eliminate the excess OPTIONS and PROPFIND to non-webdav servers
requests so our users are not continuously prompted for passwords?

Thanks,
Mark