For the first year of its availability, Windows 10 is available for free to most Windows 7 and 8 users, and Microsoft has been trying to coax those users to make the switch by delivering the operating system through Windows Update. Until now, the OS has been delivered as an optional update; while Windows Update gives it prominent positioning, it shouldn't be installed automatically.

This system has already generated some complaints, as Windows Update will download the sizeable operating system installer even if you don't intend to upgrade any time soon, but, over the last couple of days, the situation seems to have become a little more aggressive. We've received a number of reports that people's systems are not merely downloading the installer but actually starting it up.

Our own testing shows that, yes, the optional update is getting chosen by default, and that's not supposed to happen to optional updates.

For those not wanting to make the switch to Windows 10 just yet, all is not lost; the installer does require human intervention to actually proceed, so you won't run the risk of waking up to find your PC running a different operating system. If you're not paying attention, though, you may find yourself upgrading sooner than you expected.

We've asked Microsoft what's going on, and the company tells us that enabling the update was done accidentally: