No didn't try. But in case anyone is interested I was able to install and
use one of the freebie S/MIME compliant Secure Email Certificates from
Comodo. Works like a charm in Windows Mail, both for signing and encyrption.
I'm running GNU4WIN on Vista build 5456 right now with no problems.
Although I have to use WinPT to sign/encrypt my messages, because I'm
running it with Outlook 2007 Beta. So, you may have better luck then I do.