> I was able to reproduce the error dialog by changing the owner of the "Installer" folder to the current user and removing the two DACL entries that grant access to Administrators and SYSTEM.

Yeah, I'd assumed the most likely cause was the ACLs having been reset on the machine in question, though I'd expect that to cause most installs to fail. It might be that Python is the first/only MSI that the user has tried though?

It sounds like just resetting the owner isn't enough on its own, but the inherited ACLs should include SYSTEM and not prevent it from working. So I wonder if something else is at play?

Also, did your change reproduce the "Error: 0"? Or did you get an actual error code?