A:Answer You can't install Windows on the main internal hard drive. The easiest way to use Windows on the Atari is to put it on a flash drive or external hard drive. When the Atari VCS boots up, it looks for a bootable usb drive, before booting up the normal Atari VCS console.
If you'd prefer it to be internal, you can add a m.2 ssd, and put Windows on that, but then you'd have to hit f12 when the Atari boots up to choose which internal hard drive you'd rather boot from (main drive for Atari console, or added internal drive for pc mode).
I've been using a flash drive with windows for basic computer tasks and retro computer gaming, and thats been working great, and it's simple.