User-defined administrator account does not load.

Lenovo T400
Windows 7 Pro 64-bit SP1
Intel Core2 Duo P8880 2.67GHz
4.00 GB RAM

User-defined administrator account does not load at boot.
Have 2 options on startup. One is a standard user, the other is "Other Users". The same options appear when you "Switch User".
After logging into the standard user the user-defined administrator account can be seen in "User Accounts" in the CP.
The built-in administrator account is disabled and cannot be enabled, even in safe mode, as it has to be done from an administrator account.
When clicking "Other Users" at startup and entering the login details for the user-defined administrator account they are rejected.

Any help would be appreciated.

I think you misunderstood.

I used the U-D admin account to create the standard user account, not the other way around. I have been using the laptop for several years with only the U-D admin account, with the built-in admin account disabled. I have bought a new personal laptop and placed this one in my business premises. I created the standard user for the staff to restrict some functionality. Now only the standard user account appears at startup, even though the U-D admin account appears in the user accounts page in the CP after logging in to the standard user, and the built-in admin account cannot be enabled, even in safe mode, as the standard user does not have sufficient privileges.