I've had that little Windows icon sitting in my system tray for about three weeks now, the one that's supposed to make me want to upgrade to Windows 10. It doesn't make me want anything except to close it and get back to work. So this week I finally sat down and got rid of it properly, and figured I'd write up exactly what worked, because half the "fixes" floating around right now don't actually stick past the next patch cycle.
Quick background if you've somehow missed this: Microsoft pushes something called GWX (Get Windows 10) through Windows Update, it's KB3035583 if you want to go hunt it down in your update history, and it plants a little white-and-blue flag icon that reserves your free upgrade whether you asked for it or not. My mother-in-law's laptop had itself scheduled for an upgrade last week and she has no idea how that happened. She just kept clicking the X to close the popup, which, it turns out, doesn't cancel anything. Closing the window isn't the same as declining the offer. That's genuinely bad design, and I don't care how many people are happy with Windows 10 (plenty are, it's a solid release), the way this got pushed onto machines was sneaky.
Uninstalling the update itself
The most direct fix is going into Control Panel, then Programs, then View installed updates, searching for KB3035583, and uninstalling it. That kills the tray icon almost immediately. The catch is Windows Update just turns around and reinstalls it again unless you hide it too, which means right-clicking the update in the "important updates are available" list and choosing "Hide update." I did this on my desktop about ten days ago and it hasn't come back, so it actually seems to hold.
The registry route, if you want it gone at the root
If you're comfortable in regedit (and if you're not, skip down to the next bit, don't go poking around in there if you're nervous about it), navigate to:
HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\Gwx
and add a DWORD called DisableGWX set to 1. That tells the whole apparatus to leave you alone at a policy level, so future update cycles respect it too. It's the same key that Group Policy edits under the hood if you've got Windows Pro and access to gpedit.msc, under Computer Configuration, Administrative Templates, Windows Components, Windows Update, "Turn off the upgrade to the latest version of Windows through Windows Update."
Or just use somebody else's tool
If regedit makes you anxious, Steve Gibson's GWX Control Panel is honestly the easiest route I've found. It's a tiny free executable, no installer needed, and it flips the switches for you with plain-English buttons like "Monitor Windows Update" and "Prevent Windows 10 Upgrade." I ran it on my mother-in-law's laptop over the weekend and the whole thing took maybe ninety seconds, including the part where I explained to her what I was doing and why she should care.
None of this means Windows 10 itself is bad. I've been running it on my main desktop since launch week and Edge is noticeably faster than Internet Explorer ever managed to be, though I still keep Chrome open for anything that needs an extension I actually rely on. It's the forced-upgrade behavior specifically that bugs me. There's a difference between "here's a great free upgrade, come get it" and quietly reserving a slot on someone's machine while they're trying to close an ad they didn't ask to see. Microsoft apparently figures this is fine because the end goal, more people off XP and 7, is good for everyone eventually. Maybe it is. It still isn't how you're supposed to treat someone's computer without asking first.
Also, completely unrelated, but Google turning itself into this "Alphabet" holding company last week is one of those things I keep meaning to have a real opinion on and don't quite yet. Ask me again once I've seen an actual annual report with the letter A stamped on it instead of a press release.
Anyway. If that tray icon is bugging you too, hide the KB, flip the registry key, or just grab Gibson's tool and let it do the work. Ten minutes, tops, and you get your desktop back to yourself.