For years, Windows XP has trailed Windows 7 alone in worldwide usage, while the contentious Windows 8 and 8.1 struggled to hit double digit percentages. But now, two full years after the release of Windows 8, the tide appears to to finally be turning, with "the Windows 8's" cracking 15 percent global market share in October, while usage of the now-dead Windows XP plummeted.

That's according to the latest operating system stats from NetMarketShare's worldwide market share count for October, which is based on browser hits.

Why this matters: There's nothing Microsoft would love to see more than people moving off of Windows XP and adopting the company's latest platform. Finally, this appears to be happening (though Windows 7 still holds the crown and in fact grew in October). But with Windows 10 on the horizon for 2015, Windows 8.1's growth may soon halt faster than you can say, "Windows Vista."

Counting hits

NetMarketShare's OS market share graph for October 2014.

The latest count from NetMarketShare puts Windows 8 and 8.1 at 16.8 market share combined for the month of October, while XP sat just barely ahead of that at 17.18 percent. Compare that to September, when NetMarketShare had Windows 8 and 8.1 at 12.26 percent and Windows XP was way ahead at 23.87 percent.

Windows 7, meanwhile, remains at the top of the heap with more than 50 percent market share for October.

Rival Statcounter has a more conservative take on the numbers, with Windows 8 and 8.1 at 15.25 percent worldwide market share for October compared to 14.25 percent in September. Windows XP, meanwhile was sitting at 16.05 percent similar to NetMarketShare's numbers. But oddly, in September, Statcounter reported XP market share was even lower at 13 percent suggesting yet another inexplicable surge in XP market share.

Why is there such a big difference between the numbers from the two firms? Statcounter and NetMarketShare count usage stats in very different ways—raw page views versus unique users, essentially.

Windows 8.1. Why now?

The most likely reason for the dramatic uptick in Windows 8 and 8.1 market share is the back to school season. PC makers often see a boost in sales during the end of the summer and early fall as students and teachers gear up for the academic year.

It's unlikely, however, that this little surge means the appeal of Windows 8 is growing. Most new laptops and desktops are shipping with Windows 8.1, by default and the small number of Windows 7 devices still available in the consumer market will soon disappear.

Thus the increase in Windows 8.1 is likely due to the fact that the latest devices happen to be new devices running Microsoft's newest OS, rather than any inherent consumer desire for Windows 8. But hey: Microsoft will no doubt take the increase any way it can get it.