Enabling Windows to run super well on the ARM architecture is a significant engineering task. Microsoft undertook this work because when you look to the future you can see that so many of the capabilities that have been added to Windows over the years are things that customers will inevitably desire or require in the types of devices supported by today's ARM-based products-changes in form factors and the desire for mobility only add to the scenarios and capabilities we all desire in our search for no-compromise PCs.
While it is tempting to make bold statements about "starting over," we believe in the evolution of technology assets when the foundation is strong. The foundation of Windows, the core, is the most solid, scalable, and secure one around.
The video below will demo Windows 8 on an ARM Processor, as well as Office 15 and a few apps.
More on : http://blogs.msdn.com/b/b8/archive/2012/02/09/building-windows-for-the-arm-processor-architecture.aspx