Andy's Tech Experiments Blog presents a video presentation covering an experiment involving an upgrade of every major version of Windows.
According to Andy, the whole procedure took 3 days, of which approximately 12 hours was spent doing the actual installing. The remaining time was spent trying to obtain valid copies of windows. Windows 98 was installed by renaming system files to trick it into upgrading.
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Andy's Tech Experiments Blog Chain of fools: Upgrading through every version of Windows