The market researchers at IDC claim that sales of Android-based tablets will surpass those of Apple’s temporarily dominant iPad in 2013. But both systems will also lose a bit of market share in the next four years to a surprising new entry: Windows. And when you factor in PC sales, Windows continues to dominate the competition.

IDC has adjusted its forecast for tablet sales in 2013 from 172 million units to 191 million units thanks to an expected surge in lower-cost devices. The big winner this year—and going forward—will be Android, which surpassed the iPad in late 2012 and will stay ahead for good into the foreseeable future. According to IDC, device makers will sell about 93 million Android-powered tablets in 2013, good for almost 49 percent of the market. Sales of Apple’s iPad, meanwhile, will hit about 88 million units, or roughly 46 percent.

Sales of Windows-powered tablets are expected to be pretty anemic in 2013: about 5.3 million units running Windows 8 (or 2.8 percent of the market) and about 3.6 million Windows RT devices (or about 1.9 percent). It’s unclear where IDC draws the line between tablet and PC, of course, and hardware makers are expected to ship almost 350 million PCs—many of which can be described as tablets—this year as well.

But the next four years will see some interesting changes, IDC claims.

“Longer term, both iOS and Android will eventually relinquish some market share to Windows-based tablets, with Windows 8 predicted to grow from 1 percent of the market in 2012 to 7.4 percent in 2017,” IDC says. “IDC expects Windows RT growth to remain below 3 percent during the forecast period.”

If those numbers hold true, hardware makers will ship over 36 million Windows-powered tablets in 2017—split between 26 million running Windows 8 and 9.5 million running Windows RT—with growth in Windows 8 devices hitting almost 50 percent over the five-year period between 2012 and 2017.

IDC does believe, however, that Microsoft should kill Windows RT and just focus on Windows 8.

“Microsoft’s decision to push two different tablet operating systems, Windows 8 and Windows RT, has yielded poor results in the market so far," IDC’s Tom Mainelli says. “Consumers aren’t buying Windows RT’s value proposition, and long term we think Microsoft and its partners would be better served by focusing their attention on improving Windows 8. Such a focus could drive better share growth in the tablet category down the road.”

IDC had previously issues its expectations for PC sales over the 2013 to 2017 time frame, and it might be instructive to see how their predictions for a combined market of PCs and tablets changes over time. Here, I will assume that Windows continues to control about 90 percent of the PC market and use IDC’s figures for the rest.

In 2013, PC/device makers will ship a combined 320 million Windows-powered PCs and devices compared with 93 million Android devices and 88 million iPads. By 2017, Windows unit sales grow to about 380 million units, compared with about 161 million Android devices and 152 million iPads.

So much for the PC apocalypse.