Windows on ARM isn't new: from Windows Phone to Windows RT to Windows IoT, Microsoft has had multiple systems that take Windows beyond the familiar Intel and AMD processors. Older versions of Windows ran on PowerPC, Alpha, Itanium, and MIPS, after all, and in 2009 an unofficial internal project had Windows 7 running on ARM.
Development continued for ARMv7 32-bit processors with VFP floating point, NEON (ARM's version of Intel's SSE instructions for processing data in parallel) and the Thumb-2 instruction set.
But when that shipped as Windows RT, it only ran apps that had been specifically written and compiled for ARM using only the WinRT APIs. The idea was to turn Windows in a OS that was designed for mobile -- like iOS -- to get better security and battery life. But not running standard Windows programs -- either recompiled or in emulation -- was an artificial limitation (although designing Windows RT devices just for Store apps meant they used rather underpowered Tegra SOCs that couldn't have delivered good emulation performance anyway).