Microsoft (Nasdaq: MSFT) said on Friday that Windows 7 has sold 350 million licenses, which is about twice the volume of Windows Vista in the same time frame. Windows 7 has saved Microsoft from a potential Vista disaster, but challenges remain.

Windows 7 has been available for just about 18 months (if we ignore the presale event from June 26 to July 11, 2009), and it appears that Microsoft can keep up a fast pace of Windows 7 sales. On average, Microsoft has sold about 19.4 million licenses per month, and if we include the presales period, we're still looking at 18.4 million licenses per month.

We tried to find some comparison numbers for how Microsoft operating systems sold in the past and found that Windows XP sold just 17 million units in the first two months after launch and about 46 million units within 10 months after launch. Windows 95 sold about 19 million units in its first entire year of availability (1995). Windows 3.11 reportedly sold about 34 million copies over its lifetime. Of course, the PC market was far smaller back then, and the success of Windows 7 also reflects the massive growth of the PC over the past two decades.

We know that Windows Vista sold about 180 million licenses within 18 months after launch (not including three months of Express upgrade sales in fall of 2006), which gives Windows 7 roughly a 2:1 edge. A direct comparison isn't exactly fair, as the PC market has grown since the launch of Vista in early 2006. However, we also remember that Windows 7 crossed the 150 million mark nine months after launch in June 2010, and it appears that the sales pace has accelerated since then: While the first nine months saw sales of 150 million, the second nine months reached 200 million. A total of 175 million Windows 7 licenses were confirmed as sold 10 months after the operating system's launch. Windows 7 is clearly doing well.

However, it should still be noted that Windows XP still holds the majority of the OS market -- more than 50% according to Net Applications, and about 47% according to StatCounter. At the current pace, it may take another 12 to 18 months until Windows 7 will be able to surpass Windows XP as the world's most popular operating system. By then, we're already in the time frame for the release of Windows 8. Counting down the usage of software it loves to hate has become a tradition a Microsoft. IE6 has its own counter, and there's now a counter for Windows XP as well (unfortunately, that counter works only on Vista and Windows 7). We doubt that those counters will accelerate the death of those programs.

Microsoft also highlighted the success of IE9 without offering any download or usage numbers. We previouslyran an article in which we estimated the usage share of IE9 on Windows 7 at just over 7%, while the overall market share appears to be stuck below 3%. IE9 can't quite follow the success of Windows 7 yet.

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