By default when using installing the Desktop player it is only installed for the user running the installer.
To make it available to all users (for example when an admin needs to install for all users), the installer should be started from the command line with the
AllUsers=true option, e.g:
desktop_player_installer.1.8.21.release.exe AllUsers=true
When installing ensure when installing the "Register .zat and .zatx file extension" option is ticked.