If you’re using Windows 7 or 8.1, and you’re sick of being nagged by Microsoft’s pop-up to upgrade to Windows 10, go to the Ultimate Outsider website and download and install their GWX Control Panel. It’s received rave reviews. Cost: gratis. Here’s the full description.
New and Improved Method
Update, April 3, 2016: Steve Gibson, founder of GRC (Gibson Research Corp), has written a great little freeware utility that also blocks upgrades to Windows 10. Steve writes most of his code in assembler, so his utilities are tiny. He calls this newest utility Never10. He’s created a page dedicated to Never10, where you may download it for free. It’s only 81 kilobyes in size and doesn’t require installation on your Windows PC. You need just run it once to turn off upgrades to Windows 10, and run it again to allow upgrades to Windows 10. Short and sweet, it’s just what the doctor ordered.