Not so much a problem because I've solved it. Just wondered if anyone had any idea as to the cause.
Three weeks ago I reformatted my hard drive and reinstalled nearly all of my applications, but with five Photoshop plugins the installer would not work. It did start because task manager showed that setup 32.exe was running. OK, these are quite old - ten to twelve years, but I like them and they installed ok the first time.
After pondering for three weeks I solved it by rebooting into safe mode, and installing from there went perfectly, and all work fine when back in Windows proper.