When you try to add a Windows 2000 Professional computer to a Windows NT 4.0 domain, you receive:

Your computer could not be joined to the domain because the following error has occurred:
The security database on the server does not have a computer account for this workstation trust relationship.

You must have administrative privileges in the Windows NT 4.0 domain.

Even if you are a member of the Domain Admins group, you will get this error if your user name contains a space.