billie, have you tried the NT Auto Logon feature ? This auto registers the user based on their NT profile and by-passes any further registration requests.
you should make sure that peoples browsers include your company's domain in their browser as a local intranet.
i.e. On the bottom right of this browser you will see the logo and name (Internet). Double click on it. In the setup dialog box click on local intranet.
Then click on Sites... and then on Advanced...
Add your zone i.e. *.your_company.com
The user of that browser won't need a domain name or password in the future, its now a trusted site.
FYI In my network, leaving the domain blank and just filling in User/pass info works fine.