Recently, I had bought a Mac Air and I wanted to install Windows for specific reasons. There are multiple ways to install Windows OS on iOS.
1. Make a partition and install Windows as a separate OS, which means that boot loader would give you an option to choose to start either MAC or Windows.
2. Use Virtual Machine. This can be achieved using Virtual Box from Oracle (free), or Parallel Desktop.
Without doing much of research, I have ended up choosing the Virtual Box option to try it out. Because of few simple reason. The above Option 1 would have a problem of switching between the OS (iOS and Windows) without restarting your computer. And the Parallel Desktop, obvious reason to eliminate this option is is due to not being a free download.
I was so amazed to successfully compete the Windows installation on MAC in couple of minute, I mean literally in couple of minutes. So hats-off to Virtual Box.