SYMPTOMS:

Once you change a computer's membership from a domain to a workgroup and reboot the PC, you cannot log on with your previous user name and password. You may also get the following error message:

The system could not log you on. Make sure your user name and domain are correct, then type your password again. Letters in passwords must be typed using the correct case. Make sure that Caps Lock is not accidentally on.

CAUSE:

This action happens because the user name and password that you previously used are available for logon only to a domain. Your PC is no longer a member of a domain. Therefore, your logon try must be validated on the local PC by using the local security database. You must provide a user name and password that exists in the local computer's user database or Security Accounts Manager (SAM), to log on to the computer.

RESOLUTION:

To stop this activity, make sure that the local security databases on the PC have the default Administrator account from when the computer was first installed, together with any extra user accounts that have been created, in its local user’s security settings.

You cannot log on to the PC if you don’t know the local user account and password. Use the following methods to solve the problem.
For Microsoft Windows NT 4.0

Repair the registry files on the system use a latest copy of the Windows NT Emergency Repair Disk (ERD). This resolution lets you repair the computer's user database or SAM to a version for which you have a accurate user name and password.

When you change the SAM from the ERD to be conscious, this replaces the user accounts and passwords with those that existed on the date that the ERD was created. Therefore, when you use an account that was created by using this technique, you have to know that account's password on the date that the ERD was created.

Also, depending on the options that were used when the ERD was created such as whether the /S switch was used or not, the ERD may not include all existing user accounts.

By using the ERD, you are also necessary to start your PC from either the Windows NT 4.0 CD-ROM or from a boot disk, and to choose the Repair option. During the repair process, choose only the Registry option in the first window. Afterward in the repair process, a second window will recommend options for which registry hives to repair. Choose the SAM option.

For Windows XP or for Windows Vista Use a System Restore Point to restore the PC to the time at which the PC was added to the domain. Then, log on to the domain. You must know the user name and the password for an account in the local Administrators group.

WORKAROUND:

If you do not have an ERD, you can work around the problem by installing a parallel installation of Windows XP, Windows 2000, or Windows NT. A parallel installation lets you log on to the PC. Therefore, you can gain access to the PC or copy files from it. However, this technique will not let you recover either user names or passwords from the original Windows XP, Windows 2000, or Windows NT installation. Therefore, it is better for you to know a user name and password for a member of the local Administrators group before you change a computer's membership from a domain to a workgroup. you can reformat the hard disk and then reinstall Windows XP, Windows 2000, or Windows NT, Once you have backed up all the files by using the parallel installation. Or, you can configure the new parallel installation for your requirements and use it instead.