Winnt.exe is a 16-bit program and is NOT designed to perform the upgrade.
Run Winnt32.exe from within Windows NT, or W9x, to perform the upgrade.
If you try to upgrade by booting the CD-ROM or setup disks, you are not given an option to upgrade.
Winnt32.exe also supports the YES setting of the NTUpgrade option in the Unattend.txt file.
NOTE: While running Winnt32.exe, DO NOT Cancel a Save Document? prompt. If you do, the system will restart and you won't have the chance to save your data.