Regardless of what you type at the logon prompt, it continually re-displays.

This can be caused by an invalid registry entry, a type REG_DWORD <No Name> value name, which is set to %SystemRoot% at:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\Environment

To resolve the problem, boot and an alternate install of Windows NT and use Off-line Registry editing to delete the <No Name> value name.