After switching "limit local account use of blank passwords...." to disabled
in Local Security Settings I am again able to log into a machine over my
lan. How do I determine which user accounts have blank passwords so I may
change them ?

The problem seems to have started when I use "Net User" on the command line
to make sure I HAD passwords on some of these accounts ....

Try this.
http://www.bindview.com/Support/RAZO...mp2_readme.cfm

The output about the users have a blank password like the following.
name:1001:AAD3B435B51404EEAAD3B435B51404EE:31D6CFE0D16AE931B73C59D7E0C089C0:
::

Kind regards,
Urity
http://www.securityfriday.com/products/ninjutsu.html