Thanks Michael, that is encouraging to hear!
I'm pretty sure I am logging in to DNN (Windows Login) correctly. If I enter a wrong username/password, I get a Login Failed message. But if I enter my correct one, I don't get the Login Failed message, I get the Critical Error (listed earlier in this thread).

It looks as though I am getting the site settings correct, because I get a green response when I enter and hit update, which says:
Accessing Global Catalog:OK
Checking Root Domain:OK
Accessing LDAP:OK
Find all domains in network:3 Domain(s): co.santa-cruz.ca.us (CO), dcss.co.santa-cruz.ca.us (DCSS), hra.co.santa-cruz.ca.us (HRA)
(One thing I'm not entirely sure of, is the Username/password  in Site Settings. I entered my own username /password from the existing AD system. Is that correct?)

My username was created automatically in DNN the first time I successfully logged in, It created a user with my correct name from our AD, inside the DNN intranet, so something is working. So far so good.
Regarding the error, I've googled it a bit and seen that some with the same error updated the enablePassword and passwordFormat attributes in web.config. What are your settings for those? (This didn't work for me, and I'm not sure if it's safe to alter those settings).

Since you're successfully using the AD Authentication Provider extension, I have a few more questions if you don't mind:

Can your users automatically log in? By this, I mean, if they are already logged into your organization's network, and they browse to your DNN intranet, are they logged in, or do they need to enter their username/password again?

When their password is changed in AD, does it get automatically updated in DNN?

Is this module still supported? When was the latest update? Is there any documentation out there?

Thanks again Michael!

-Tom

Still trying to get this working. What I meant in my last question was, Which Password settings are you using in your web.config