How to install AviSynth+ 32-bit? I have used and installed AviSynth+ 64-bit on a machine. I then wanted to install AviSynth+ 32-bit to use the watermark filter. I followed these instructions to install 32-bit, changed the registry keys, and got a blue screen of death.

After a fresh reinstallation of the Windows system, I want to install 32-bit AviSynth+ only and avoid another blue screen of death.

On the new version 3.5.1 of AviSynth+, is XP the 32-bit version, or is any installer able to install a 32-bit version?
There is no problem with having installed both 32- and 64-bit version of AviSynth on your 64-bit machine, if you correctly placed 32-bit plugins vs. 64-bit ones to their appropriate places.

AviSynth is a frame server, it means, you never call it directly - other program call it back. It means that for example


64-bit Media Player Classic will call the 64-bit version of AviSynth,
32-bit Media Player Classic will call the 32-bit version of AviSynth.


Similarly


64-bit FFmpeg will call the 64-bit version of AviSynth,
32-bit FFmpeg will call the 32-bit version of AviSynth.


I guess that your problem lies in your FFmpeg installation — you have probably 64-bit, and it means that in the process of converting your .avs script to some media file the FFmpeg calls 64-bit version of AviSynth, and you need the 32-bit (because of your 32-bit filter).

So my advice is, install both 32- and 64-bit versions of FFmpeg (into different directories) and use the appropriate one for every particular case.