But during the first two months of Windows 7 PC sales, the Index was even lower than currently, with an average of just 49.7 for the final two months of 2009, when the U.S. was in the throes of a major recession after the financial implosion in 2008. According to the Index, Windows 8's uptake should be greater than Windows 7's. It's not.
Instead of leaning on only upgrade tendencies and economics, most analysts have hammered a different observation: Unlike its two precursors, Windows 8 -- and the PCs that run it -- must compete with tablets, a device category jump-started by Apple's iPad in April 2010.
Under that explanation, dollar defections to tablets translates into a slow-down in PC sales -- and corresponding sluggishness in new operating system's uptake.
It's clear that users, whether consumers or enterprise workers, are increasingly choosing tablets -- virtually all of which run a non-Microsoft OS -- as their mobile device. That reduces sales of Windows machines, especially notebooks, and generally lengthens everyone's PC refresh cycle.
Research firm IDC, for instance, recently raised its 2012 tablet sales forecast to 122 million devices, up 72 per cent from 2011. Meanwhile, sales of PCs -- although still dwarfing tablets -- have stalled, and will likely come in flat for the year or even down from 2011.
Last October, IHS iSuppli forecast the latter, noting that if its projections turned out accurate, 2012 would be the PC market's first annual decline since 2001, when the "dot-com" bust traumatized the industry.
Net Applications' numbers also reported on trends other than Windows 8's uptake.
Windows XP lost seven-tenths of a percentage point last month to close the year at 39.1 per cent of all personal computers, or 42.6% of Windows-only machines. Vista slipped slightly as its share continued to edge toward zero.
Meanwhile, Windows 7 picked up four-tenths of a percentage point, ending December with a 45.1 per cent share of all PCs and 49.2 per cent of all Windows PCs.
Windows overall gained ground last month, increasing its share by three-tenths of a percentage point, the largest increase since last March. At the end of 2012, Windows powered an estimated 91.7 per cent of the world's personal computers, down half a point for the year, just a third of Windows' loss during 2011.
Mac OS X and Linux both lost share in December, with the former down two-tenths of a point to 7.1 per cent and the latter off less than one-tenth of a point to 1.2 per cent. Apple's operating system finished the year up seven-tenths of a percentage point, while Linux was down two-tenths of a point.
Net Applications measures operating system usage by tracking unique visitors to approximately 40,000 sites it monitors for clients.
Sign up for Computerworld eNewsletters.