Microsoft is trying to push Windows 10 to all directions, including embedded systems and robotics, in their Build 2015 event. Windows 10 is now available for the Raspberry Pi 2 and Intel Minnowboard Max. To demonstrate that could be done Microsoft also showed off a Raspberry Pi-powered robot during the Build keynote.
Raspberrypi.org blog has posting Windows 10 for IoT that tells Windows 10 for IoT to the Raspberry Pi. The Windows 10 for IoT posting has a ridiculously cool demo video that combines Windows 10 for IoT to the Raspberry Pi powered robot and HoloLens:
Downloading the Windows 10 Core IoT Preview (pre-release software) is all free, but you need to have to have a copy of Windows 10 on your PC to use it.