I need to authenticate against existing user accounts within Windows (e.g., a local account on the web server, or a domain account in the active directory, etc.).
The catch is:

1. I need mixed-model authentication.
Some logins will be part of the company infrastructure, and some won't (because the people will be external to the company). I don't want to add the admin burden of getting those external people into the active directory. Therefore, those users should have an internal username and password preferably on a SQL backend.

2. I want a single login page.
I would like to use a single form to log everybody in, when needed.

3. I would like seamless login for domain users
It would be nice if I could get seamless login for domain users, and have them not even have to fill out the login form.

I've searched around and haven't found a sample that addresses all of this. Is this wishful thinking or can this be done?
TIA