In the first in a short new series of articles examining the ever-changing personal computing landscape, I take a look at where Windows sits in today's world, and where it will likely sit in the near future. This evolution clearly explains why Microsoft needs to support other devices with its services.

So here's the harsh reality: Windows just isn't at the center of everything anymore. That's not the future. It's the present. And that's why folks like Steve Jobs have referred to us being in a post-PC world. (Well, that and self-serving reasons like Apple doesn't dominate the traditional PC market.)

As Microsoft recently discussed at its Financial Analysts Meeting (FAM), Windows is now the firm's third-biggest market, behind Office and Server. But we have better proof of changing trends than that. Looking at IDC expectations for device sales in the 2013 calendar year, we see the following:

PCs: 315 million (desktop PCs: 134.4 million, portable PCs: 181 million)

Tablets: 227.4 million

Smart phones: 1 billion

So of the approximately 1.5 billion personal computing devices that will be sold this year, only one-fifth of them are PCs. And while Windows currently dominates that part of this aggregated market, it's barely even a player in the rest of it.

Of course, smart phones and PCs are not directly comparable, and many tablets are used only for consumption-style activities and not for email, web browsing, or whatever. But things are changing.

Looking to 2017, IDC expects over 2.5 billion personal computing devices to be sold. But only tablet and smart phone sales will rise appreciably. That market is predicted to break down like so:

PCs: 319.7 million (desktop PCs: 123.1 million, portable PCs: 196.6 million)

Tablets: 406.8 million

Smart phones: 1.73 billion

In this new world, PCs are only one-eighth of the entire market for personal computing devices. On the flipside, PCs haven't disappeared. Indeed, sales of portable PCs have risen, though it's also worth pointing out that tablets will outsell portable PCs by 2:1.

These changes explain Microsoft's emphasis on the touch-first Metro environment in Windows 8.x, and on pushing Windows and Windows Phone closer together. But it's not clear what Microsoft could do to fix the central problem with Windows: It's just not dominant anymore.

Let's do some math to see why.

Being overly favorable to Microsoft, let's give them 90 percent of PCs sales in 2017, and assume the rest goes to Mac, Chrome OS, Linux, whatever. That's 288 million units. Let's further assume that Windows splits the tablet market with iOS and Android—a ludicrous assertion, but this is for best-case scenario purposes—and nets another 135 million units. And let's say that Windows Phone is able to snag 10 percent of all smart phone sales, which is about triple the current share: That's 173 million units.

Add those inflated numbers up and you get 596 million units, or ... wait for it ... a bit over one-fifth of the expected market size for personal computing devices in 2017. Even with these overly-inflated numbers, Windows only accounts for one in five devices being used worldwide. And that's never going to happen.

So what's really going to happen?

Today, Android controls 63 percent of the tablet market, and that figure will only grow going forward, to about 75 percent in 2017. Apple's iPad is of course number two, with 32.5 percent share, and Windows is a distant third with about 4.5 percent. But Windows is expected to have 10.1 percent market share in 2017 and iOS will have 13 percent.

Android controls 75 percent of the smart phone market, and that figure is expected to end up at about 68 percent by 2017, with iOS coming in second with 18 percent and Windows Phone at 10 percent.

So. It's 2017. Here's how all that share breaks down:

PCs. Windows controls 80 percent of the market for PCs, or 256 million units. Android and Apple (Mac) split the rest, mostly Apple (16/4 percent, I bet.)

Tablets. Android's 75 percent share is 305 million units. Apple's 13 percent is 53 million units. Windows' 10 percent is 41 million units.

Smart phones. Android's 68 percent of 1.73 billion is 1.176 billion units. Apple's 18 percent is 311 million units. And Microsoft's 10 percent is 173 million units.

Add it all up, and you get this:

Microsoft/Windows: 470 million units.

Apple iOS (iPad/iPhone): 415 million units

Google/Android: 1.49 billion units

So what we see here is that the future of the personal computing market is basically Android overall, from a volume standpoint. Windows and iOS are still big, and popular, and viable markets no doubt. But the mainstream computing environment is dominated by Android, which wins big in the two biggest of the sub-markets, tablets and phones.

And before we go, yes, this is all speculative. Yes, the math could be off. But I think the broad strokes are representative of where things are going. Which is why we're having this conversation in the first place.

Next: Considering Microsoft's role in the post-PC world.