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Windows 8 will be the first full blown version of Microsoft’s desktop operating system which can run on ARM-based processors as well as x86 chips. But Windows on ARM will be a different experience for many users — because most older Windows apps aren’t compiled to run on ARM chips.

In other words, moving forward we’ll probably see key apps such as Microsoft Office, Mozilla Firefox, and Skype supporting Windows on ARM as well as Windows on x86. That’s because these apps are under active development and it makes sense for the companies behind them to ensure that they can run on all versions of Windows.

But there’s no guarantee that your favorite software which you’ve been running for the past 10 years… and which hasn’t been updated in the past two years will run on a new ARM-powered computer.

And that’s at least part of the reason why NVIDIA CEO Jen-Hsun Huang says Microsoft shouldn’t position ARM-powered Windows devices as PCs — because they might not act the way people expect PCs to act.

Over the last year or two NVIDIA has become one of the most important makers of ARM-based chips thanks to the company’s Tegra line of processors which currently power a number of Android smartphones and tablets. Future NVIDIA Tegra chips will also be able to support Windows 8, so we could see tablets and possibly laptop or desktop computers running Windows with NVIDIA processors in the future.

Huang is probably right, in that it’s important for Microsoft to minimize consumer confusion about different devices.

At the same time, this sort of move could be frustrating as all get-out because the distinction isn’t all that clear. Nobody really expects an Apple iPad to run OS X apps — because it’s not a Mac. Sure, there are versions of the Safari web browser for both platforms, but fundamentally it’s very clear that a Mac and an iPad are running different operating systems.

The same will not be true of Windows 8 tablets — because Windows 8 will be able to run on devices with a variety of form factors including desktop, laptop, and tablet computers with a wide range of chips. It will essentially be the same operating system on all of those platforms — it’s just that if you have an ARM-based chip you’ll probably get better battery life and you won’t be able to run as many apps.

Odds are most of the ARM-based Windows 8 systems, at least initially, will also use the new Windows 8 Metro style user interface for full-screen, finger-friendly apps. But you’ll be able to minimize that UI and run full-blown Windows apps on any Windows 8 device, as long as the apps support ARM architecture.

Over the past few years I’ve gotten used to explaining to people that netbooks are just like any other laptop — they can run Windows and therefore they can run almost any app you would run on your full sized computer at home or at the office. They just might not run it quite as well because of the slower processor or smaller screen (if someone doesn’t get that a netbook is a computer, it’s tough explaining that the screen resolution is more important than the size).

If Microsoft and its partners really do start to market Windows 8 tablets with ARM-based chips as a separate class of machines, I’m going to have a much harder time explaining what it means when a device runs Windows instead of Windows CE, Android, or another operating system.

So while Huang is probably right that it’s important for Microsoft and other companies to manage user expectations, for entirely selfish reasons I kind of hope nobody decides to say that ARM devices aren’t PCs… because it’s hard enough explaining what a computer is already.