/me not so idly uninstalls THE FUCK out of Windows update KB3035583.
— Kane Magus (@KaneMagus) June 1, 2015
If you're running Windows 7/8 and see a little icon in your system tray that looks like the Windows 10 logo, that's thanks to Microsoft update KB3035583, which installs nagware for upgrading to Windows 10. If you don't want that shit, uninstall KB3035583, and then hide the update in Windows Update (where it is scuzzily marked as an "Important Update").
(EDIT 02-02-2016) Actually, it was probably just a "Recommended update" and I didn't realize it until later, after I had unchecked that fucking "Give me recommended updates the same way I receive important updates" bullshit. Still, the fact that it was showing up as "Recommended," instead of purely optional, even as far back as May 2015 is some serious bullshit. (/EDIT)