It should actually execute Avisynth installer IF there's no "avisynth.dll" in %windir%\syswow64.
The point is if it's already installed - it's up to you to ensure if it's installed correctly.
Installer's log could help - just press "show details" button
Installing component Avisynth 2.6 32-bit
C:\Users\chain_000\AppData\Local\Temp\qtifw-jM3v2V\AviSynth_260.exe