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

The specified Setup script file (\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"