Wow, I just confirmed this myself.
My original CNC controller PC was XP. When I upgraded it to Win7 32-bit I stopped using it for my CNC machine and instead purchased a dedicated Dell Optiplex 745 running XP.
I upgraded my Win7 PC to Win10-32-bit six or eight months ago (and never tried running Mach3 which was still installed in C:\Mach3 because I had heard it wouldn't work).
As the "free" upgrade to Win10 deadline was approaching I decided I'd better upgrade my Dell Optiplex to at least Win7 32-bit. I installed Win7 OEM (which was NOT an upgrade install but a fresh new install, did the Win7 updates, then performed the Win10 32-bit upgrade. My C:\Mach3 folder and installed files were still intact. I had the ubiquitous problem with the win10 upgrade hanging on the "Checking for Updates" after downloading and verifying all the installation files. After numerous attempts and web searching to correct this to no avail, I tried the downloading the installation file set to an .ISO filetype then burning to DVD. The Win10 install completed flawlessly, along with updates.
It was only then that I tried running my mach3.exe from C:\Mach3 and it would not load my profile, would give me an error that C:\Mach3 was Read-Only, then only open a blank Mach3 app window. I re-installed Mach3 to C:\Mach3, same thing happened again. After reading that Win7, 8 and 10 uses User Access Control (UAC) to limit what can be stored in the root of C: I did the install of Mach3 to C:\Program Files\Mach3. Same thing happened as before. I must still sort out what is going on with this Dell Optiplex.
Just now, on a lark, I went back to my former CNC generic PC (which had been running Win10 32-bit for almost a year without problems) and double-clicked on mach3.exe in C:\Mach3....and it ran, opened the normal Mach3 app window. The properties of C:\Mach3 showed this folder (and subfolders) to be Read-Only (go figure).
So I have my definitive answer, Mach3 will run (and open a normal app window) on an upgraded XP to Win7 to Win10 32-bit PC (the parallel port driver working not with standing).
I'm going to try to do a fresh Win7 32-bit OEM install on the Dell Optiplex 745, with updates etc., then install Mach3 fresh to C:\Mach3. Then I'll see if it runs normally or requires UAC to be disabled. If it does run either way, I'll then try the Win10 upgrade and see what happens.
I hope this helps someone else.