If an Administrator uses the Users and Passwords tool on a Windows 2000 Professional computer to alter a user's group membership, the user may not be able to logon locally.
When you use the Control Panel/Users and Passwords tool and select a local user's Properties/Group Membership/Other/
If the group that the user is a member of does not have the log on locally right, they will be unable to log on.
Use Computer Management to add the user to groups that have the right to log on locally, and to manage local group membership in the future. You can also open a CMD prompt and use:
NOTE: If "User Name" uses the "Domain Name Domain User Name" syntax, you can add a domain user to a local group.