The installed version of REEF is the recommended version for instructors using Windows. This version includes a REEF Launcher application that allows you to start polls and quizzes from the instructor website and will make it even easier to update REEF when future releases are available. Administrative rights are needed to install the REEF Launcher; check with your IT department if you don't have these privileges for your computer. Alternatively, although you won't benefit from the REEF Launcher functionality, you can continue to download and use the non-installed REEF application without the Launcher.
Mac users don't have an installed version as the Mac OS doesn't require a separate Launcher application to perform the same functionality.