The path to the Windows NT 4.0 Setup folder usually contains the letter of the CD-ROMdrive and "I386" for "Wintel" machines. So, for example, if drive D: is the CD-ROM drive, then D:\\I386 would be the Setup path.
To change the default location of the setup files, edit the string value SourcePath found at:
HKEY_LOCAL_MACHINE
\\SOFTWARE\\Microsoft
\\Windows NT\\CurrentVersion\\Setup
So, for example, if the Windows NT setup directory changes to G:\\I386, set SourcePath to "G:" (do not be tempted to enter "G:\\I386").