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Microsoft this morning announced that Windows 7 has reached 350 million licenses sold in 18 months on the market. It’s an impressive number on the surface, but absent any other context it’s hard to get a sense for what the number actually means. Here’s my best attempt to put it in perspective based on some quick research.

Over the same 18 months, IDC reports a cumulative total of about 517 million PC shipments, based on data gleaned from the research firm’s quarterly reports. (Gartner pegs the market slightly higher, at 523 million shipments.) In other words, as a proportion of total PC shipments during the time of its availability, Windows 7 licenses are at 68 percent, based on IDC data.

[An important aside before we continue: That percentage figure doesn’t correspond to operating system market share, in part because not all computers ship with operating systems pre-installed. (Update: Continued shipments of older, non-Windows 7 versions during that time frame are a larger factor.) I’m using it as a benchmark in this case to get a sense for how Windows licenses track against the overall growth in the PC market. Windows’ overall share of the OS market is much higher.]

Is that good or bad? By comparison, Windows Vista had sold more than 180 million licenses at its 18-month mark, according to a Microsoft regulatory filing at the time (Page 23). Over the same time period, from Q1 2007 to Q2 2008, IDC shows about 400 million worldwide PC shipments. In other words, as a proportion of total PC shipments during its first 18 months on the market, Windows Vista was at 45 percent — considerably lower than the Windows 7 data announced by Microsoft today.

So Windows 7 is kicking Windows Vista’s butt. No surprise there, really.

But what about Windows XP’s first 18 months? I haven’t yet been able to find similar PC shipment and OS licensing figures dating back to early 2003, which would have been the 18-month mark for that Windows version.

However, one news story from that time notes that Windows XP had grabbed 34 percent of the market after 18 months, based data collected from Internet users. Within the same population, Internet users, Windows 7 currently has 27 percent of the market — not far behind, but not as well as Windows XP was doing at the same point in its life cycle.

More telling is that, by the same measure, Windows XP right now — in 2011, nearly a decade after its release — still has 54 percent of the worldwide market, judging from the computers people are using to connect online.

This is an imprecise exercise, a back-of-the-napkin kind of thing. Feedback welcome on my methods and reasoning.

Bottom line, Windows 7 is doing well, especially when compared with Vista. But it still needs to exorcise a certain ghost from the past.

Update: Charles Arthur at the Guardian does a nice job of picking up where I left off and finds that the comparable number for Windows XP licenses in its first 18 months was 54 percent of PC shipments — considerably less than Windows 7’s 68 percent, giving the new Microsoft operating system the best growth rate. Very interesting stuff, worth a read.

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