Here is the problem in a nutshell:

In SQL Server 2005, when I attempt to create a Login using a 'user' created account, I get a 15401 error. In other words, I can only add 'BUILTIN' accounts that Windows automatically creates as SQL Server Logins.

The exact error is:  Windows NT user or group 'TITANFCI\Domain Users' not found. Check the name again. (Microsoft SQL Server, Error: 15401)

I have been through Microsoft's "How to troubleshoot error 15401 (Article ID : 324321)" located at http://support.microsoft.com/?kbid=324321 but none of this information applies to me.  The SQL Server’s default collation is set to case sensitive (do not know why) but it does not matter what case is used to enter the SQL logins, the same error occurs.

I have been working the Microsoft Tech Support and have eliminated Active Directory as a potential problem. 

As a side note, I have SQL Developer Edition installed on my workstation (within the same some domain) and I do not have these problems.  So, I am fairly confident that the problem is specific to this instance of SQL Server installed on the Primary Domain Controller.

We are a small, single domain and single server company.  SQL Server 2005 Standard Edition is installed on the domain controller.  Unfortunately, I did not perform this install so I can not vouch for its integrity.