When you attempt an unattended upgrade of Windows NT 4.0 to Windows 2000, you receive:

The specified Setup script file (<path>\unattend.txt) is inaccessible or invalid.                              Contact your system administrator.
NOTE: This error occurs with winnt32 /unattend:unattend.txt.

You are missing the UnattendMode=FullUnattended parameter in the unattend.txt file:

   \[Unattended\]                                 NTUpgrade=Yes                                 UnattendMode=FullUnattended                                 \[UserData\]                                 ProductID="xxxxx-xxxxx-xxxxx-xxxxx-xxxxx"