With Windows 10, Microsoft has rewritten the rules for how it performs product activation on retail upgrades of Windows, including the free upgrades available for a year beginning on July 29, 2015. The net result is that clean installs will be much easier--but only after you get past the first one.

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OEM activation hasn't changed, nor have the procedures for activating volume license copies. But the massive Get Windows 10 upgrade push means that for the near future at least those retail upgrade scenarios are very important.

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The biggest change of all is that the Windows 10 activation status for a device is stored online. After you successfully activate Windows 10 for the first time, that device will activate automatically in the future, with no product key required.

That's a huge change from previous versions of Windows, which required a product key for every installation. And it's potentially an unwelcome surprise for anyone who tries to do a clean install of Windows 10 without understanding the new activation landscape.

Microsoft is characteristically shy about discussing the details of activation. That's understandable, because every detail the company provides about its anti-piracy measures offers information that its attackers can use.

But it's also frustrating, because Microsoft's customers who use Windows don't want to have to think about activation. The Windows PC you paid for, and the free upgrade you spent time installing, should just work.

I've had some way-off-the-record discussions with people who know a few things about the subject, and I've also done my own testing for the two weeks since Windows 10 was released to the public. Here's what I've learned...