Microsoft dipped its toe in the ARM waters with Windows RT but it ultimately proved a failure, primarily because of the lack of applications. That doesn't mean the company is going to give up on the dominant mobile market processor, however.
At the WinHEC show in China last week, Microsoft announced it has finally created a native version of Windows 10 running on Qualcomm Snapdragon processors with full x86 compatibility. This isn't Windows RT, it's full Windows 10, but you’ll be able to run Win32 apps and Windows Universal Apps.
The initial version of ARM Windows 10 only supports 32-bit apps, but that's not a big deal since most mobile devices have 4GB of memory or less anyway. And it should be noted this is Windows 10 on Snapdragon, not Windows 10 on ARM. Sure, Qualcomm's processors are fairly ubiquitous but they aren't everywhere. You won't be running Windows 10 on an iPad any time soon.