The free period to upgrade to Windows 10 is coming to an end in July and Microsoft has now changed how the upgrade prompt behaves.
Up until now, people who have decided not to upgrade to Windows 10 have responded to this prompt by clicking on the red X in the top right corner to get rid of it:
Now when you do this it causes you to agree to the scheduled upgrade.
It is then up to you to stop the upgrade when it starts. This has happened to a number of my clients, one while I was actually there.
As you can see, there is a way to decline but it is now in the middle of the prompt and if you’re just used to cancelling it with the X it is difficult to notice.
