Windows 8 may be something of a point upgrade for existing Windows PC users, but it could herald the birth of PCs in all shapes and sizes.

After all, Windows 8 runs on the same hardware as its predecessor, and should be a relatively straightforward upgrade. The Metro interface is its only radical change, and it is optional.

But Microsoft is releasing a whole new operating system alongside Windows 8, which has the potential to radically change Windows computing forever. It’s also called Windows, but its full title is WOA: Windows On ARM.

ARM processors have just about taken over the mobile world, and are now found in the majority of tablets and smartphones. And these product categories are growing rapidly as desktop PC sales decline.

Windows 8 is designed to be used on desktop and laptop PCs, and on mobile devices. WOA represents Microsoft’s ambition to fill the gap between laptops and smartphones with a wide variety of new types of personal computer – thin and light systems with lengthy battery life.

WOA uses the Windows 8 code base and will replicate many familiar Windows design features, such as its desktop interface. In other ways, WOA will be uniquely crafted. It will, for instance, require that devices running it use a system-on-a-chip design.

Any type of device that runs on ARM processors, from the tiniest smartphone to smart-fridges and -TVs, will be able to run Windows. And plenty will, given the popularity of Windows with the majority of the world’s computer users. We know Microsoft has been working with tablet makers on ARM-based Windows devices, for instance; expect a lot of those.

But device makers are limited by more than just their imagination. Microsoft has locked down the hardware specification of WOA devices, which extends to such things as the number and type of connectivity ports they support.

This seems like a strange decision on Microsoft’s part if it wishes to promote diversity: locking down the hardware specification of netbooks went some way to killing that breed of device (and Windows XP, which was Microsoft’s intention).

But there’s a bigger prize at stake here. Microsoft has for a long time watched from the sidelines as Apples’ App Store has gone from strength to strength. As the Android Market also develops, one thing becomes clear: if you want to sell lots of hardware, you need a viable software ecosystem. And if you want the software, you need to sell lots of compatible hardware.

It’s a tough nut to crack, but Microsoft starts from an unrivalled position: it is the dominant player in the consumer and business PC world. It also has pots of cash with which to entice hardware and software makers to commit to Windows. And it has Office – the productivity tool used by most of the world’s enterprises, and bundled in with WOA.

WOA is a statement of intent: Microsoft wants to be part of a multi-device world in which Google and Apple don’t have things all their own way. That’s good news if you like using Windows.