Microsoft's been trumpeting a pretty big number lately: 350 million, as in the number of Windows 7 licenses sold in the 18 months since that operating system hit the street.

Windows 7 was essentially a do-over of Windows Vista, which never quite managed to overcome its initial reputation as an unwieldy and bug-filled operating system. Microsoft took no chances with the rollout, accompanying it with a multimillion-dollar ad campaign and a wide variety of discounts.

But how is Windows 7 faring in the overall market? Some outside analytics firms offer some interesting statistics on that front. According to analytics firm Net Applications, Windows 7 holds 24.7 percent of the worldwide operating-system market, trailing Windows XP with 54.39 percent. Vista takes third in that reckoning, with 10.56 percent, followed by Mac OS X 10.6 with 3.50 percent.

Those numbers are broadly matched by StatCounter, which places Windows XP at 47.32 percent of the worldwide market, followed by Windows 7 with 20.6 percent, Windows Vista with 13.66 percent, and Mac OS X with 6.53 percent. (In the United States, Windows 7 at 30.84 percent seems on the verge of overcoming Windows XP at 32.17 percent.)

If anything, those numbers just go to show Windows XP's deep entrenchment among businesses and consumers, more than a decade after its release. In that time, numerous patches and add-ons have made XP a warhorse, one that Microsoft desperately wants people to abandon. Microsoft Download Center now offers a Windows XP End Of Support Countdown Gadget, which counts down the days until the operating system's official support ends in 2014. Meanwhile, Microsoft's latest browser, Internet Explorer 9, won't run on XP.

Eventually, Windows 7 will almost certainly overcome XP. Analysts seem to expect that businesses will engage in a more substantial tech refresh in coming years, once the economy stabilizes a little more, further propelling adoption of the operating system. And consumers, no matter how dedicated to XP, will eventually need to replace their aging laptops and desktops. If they want to stick with a PC, they'll need to purchase one running Windows 7.

In the meantime, though, rumors suggest Microsoft will produce the next version of Windows sometime in 2012. If that proves true, that could alter the game entirely. Would those still running XP at that point graduate directly to "Windows 8"? If Microsoft announces its next-generation OS too soon, will that curb Windows 7's healthy adoption rate? Time will tell, but for the moment, Microsoft can probably claim success for the Windows franchise--a good thing, considering that traditional platforms such as Windows and Office drive the lion's share of the company's revenue, despite all the attention focused on the cloud and smartphones.