[@Paulo A.G. Berardo de Andrade]
I encountered the same situation, and what I did was that I decided to remove the previous Zemana version using SoftOrganizer which I obtained from here (thank you Ashraf), and then proceeded with the installation of MalwareFox. It installed then without a hitch. Make sure though to create a restore point before removing Zemana, just in case something goes wrong in your particular case.
Hope it helps you and some other people.
Cheers,
consuela