Forget Windows on ARM; Windows on Sharp SM83 is the latest thing.
A developer known as Rubenretro on X has ported Windows 3.1 to the original Nintendo Game Boy DMG-001 handheld. The version of Windows, though heavily modified, includes some staples of Microsoft's early operating system, including a few tools and games.
Rubenretro launched the project, known as GBS Windows, earlier this year, selling physical copies last month. That batch quickly sold out, so the Game Boy developer released a second batch last week, which also sold out. A third batch is coming, according to @CajasRetro, the company responsible for sales and distribution.
GBS Windows was developed in GB Studio, a drag-and-drop development environment for the DMG-001 (the original black-and-green Game Boy) and CGB-001, also known as the Game Boy Color (curr. available refurbished from Amazon for $112.87).
This version of Windows, which is based on Windows 3.1, includes Minesweeper (known as GBSweeper here), a version of MS Paint (known as GBSPaint), and a media player with some included chip tunes. The media player can actually play music in the background while other apps are in use, allowing for some rudimentary multitasking. Additionally, the Game Boy Printer accessory works with GBSPaint to print out users' pixel art masterpieces.
GBS Windows was listed on Etsy via Cajas Retro, but is currently unavailable.