After you’ve installed Windows 10 without a key, it won’t actually be activated. However, an unactivated version of Windows 10 doesn’t have many restrictions. With Windows XP, Microsoft actually used Windows Genuine Advantage (WGA) to disable access to your computer. These days, Windows just complains at you in a few minor, cosmetic ways.
Initially, you won’t notice a difference. Eventually, Windows will start nagging you a tiny bit. First, you’ll notice a watermark in the bottom-right corner of your screen. You’ll also see a “Windows isn’t activated. Activate Windows now.” link at the bottom of the Settings app. This is the only form of nag you’ll see–there are no pop-up windows, for example.
Second, you’ll be unable to change your desktop wallpaper and from the Personalization > Background screen in the Settings app. You’ll see a “You need to activate Windows before you can personalize your PC” message at the top of this window, and the options for changing your wallpaper will be grayed out.
You can still change your wallpaper in other ways, however. For example, you can right-click an image in File Explorer and select “Set as desktop background.” You could also open an image in the Photos app, click the menu button, click “Set as,” and click “Set as background.” Windows 7 eventually switched you back to a black background, but Windows 10 doesn’t seem to do this.
You’ll find Windows 10’s included wallpapers under the C:\Windows\Web folder in File Explorer.
Aside from these basic limitations, your Windows 10 system will continue to work forever. There are no nag prompts aside from the watermark, you’ll get all the system updates, and everything else is completely functional. The only thing that could change this is a Windows 10 update, but Microsoft has become increasingly lenient since Windows 7.