As you may recall, Microsoft has delivered KB3035583 as a 'recommended update' to users of Windows 7 and 8.1. What this update does is install GWX ("Get Windows 10"), a program which diagnoses the system to see if it is eligible for a free upgrade to Windows 10, and if so, asks the user if they would like to upgrade (though recently, the option to decline has been removed). Some users have gotten around this by editing Windows Registry values for "AllowOSUpgrade", "DisableOSUpgrade", "DisableGWX", and "ReservationsAllowed" in order to disable the prompt altogether. This advice wasendorsed by Microsoft on their support forums.

According to a report by Woody Leonhard at InfoWorld, the newest version of the KB3035583 update includes a background process which scans the system's Windows Registry twice a day to see if the values for the four aforementioned registry inputs were manually edited to disable the upgrade prompt. If they were, the process will alter the values, silently re-download the Windows 10 installation files (about 6 GB in total), and prompt the user to upgrade.