Windows 7 will remain a widely-used OS for at least the next half-decade if its predecessor Windows XP serves as even a crude yardstick.

Five years ago last month, Microsoft moved Windows XP onto its do-not-support list. The company retired the well-worn operating system on April 8, 2014. Yet even now, 60 months after the milestone, more than 40 million PCs worldwide rely on the now-ancient software.

Windows XP's post-retirement trajectory can be instructive when forecasting what may happen to Windows 7 when it falls off the support list in eight months, on Jan. 14, 2020.

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