After a historic two-plus year drop in PC sales, the market will rebound next year, with unit sales increasing year-over-year in 2015 to 317 million units. That's the word from the industry seers at Gartner, who say that businesses are finally making the final push towards getting Windows XP off the desktop.

I was curious to see whether the positive results next year were related in any way to Windows 9 (currently codenamed Threshold), which should complete the reversal of the misguided mobile-first strategy that Microsoft employed with Windows 8. But instead, Gartner attributes the coming rebound to those XP replacements, most of which will be to Windows 7 (a.k.a. the new XP).

That said, I do expect Windows 9—which will ship in variants that are respectively better adapted to both traditional PC form factors as well as tablets, hybrids and phones—to finally get the PC industry aligned around a Windows that makes sense for everyone. Windows 8 was simply too divisive, and was dismissed out of hand by both consumers and businesses alike.

Anyway, Gartner.

The firm says that PC sales will drop modestly again this year, by 2.9 percent, to 308 million units. That's not horrible: PC sales fell 9.5 percent in 2013 (using Gartner's numbers).

That this figure is still above 300 million is surprising to me, but part of that is indeed XP upgrades, 60 million of which will occur this year alone, Gartner says. (This suggests that "new" Windows/PC sales are roughly 250 million units, which is basically in-line with what I was thinking.) But that's also a fairly tiny percentage of the overall personal computing devices market—i.e. PCs, plus tablets and smart phones—that will ship this year. Gartner says that 2.4 billion such devices will be sold this year, and 2.5 billion next year. So the PC is about 7.8 percent of the overall market now. Yikes.

Tablets, meanwhile, are expected to hit 256 million units this year, up from 207 million in 2013. But tablets will hit 321 million in 2015, allegedly, surpassing the PC for the first time.

Meanwhile, phone shipments will hit 1.86 billion units in 2014, according to Gartner, and 1.95 billion next year.

As for platforms, Android will ship on 1.17 billion devices in 2014, and 1.37 billion in 2014, while iOS and Mac sales combined will be 271 million units this year. That latter figure is still well below the combined Windows and Windows Phone sales of 333 million devices, up from 326 million in 2013. Next year is even better for Windows/Phone, though, Gartner says, with a total of 374 million devices sold, well above the iOS/Mac estimate of 301 million units. So Apple is heading downward, it seems, from a market share perspective. Nothing new there.

But here is something new: Gartner now expects Windows Phone to hit the crucial 10 percent market share milestone by 2018, thanks to strong growth. That's huge, considering that Windows Phone will account for only 4 percent of the smart phone market in 2014.