Or, hear me out, buy cheap SSD for each operating system. For as low as $200 you can have a solid operating system of any flavor. Not an option because you're running thing on laptop and is not good to tear the laptop apart to replace SSD? OK, then how about:
Run each operating system in it's own VM and you can encrypt its folder. Not an option because you want to use games on your Windows and those are notoriously difficult to run inside a VM? OK, then how about:
Run a hypervisor on bare metal, like VMWare ESXi. You get now each operating system to be running on bare metal just like if it's alone and on Windows you can definitely play those notorious difficult games.
Run each operating system in it's own VM and you can encrypt its folder. Not an option because you want to use games on your Windows and those are notoriously difficult to run inside a VM? OK, then how about:
Run a hypervisor on bare metal, like VMWare ESXi. You get now each operating system to be running on bare metal just like if it's alone and on Windows you can definitely play those notorious difficult games.