Please confirm whether your operating system is genuine because it is most likely that your Windows does not pass "Windows Genuine Advantage Validation". According to Microsoft official web page, system will start to verify your operating system after Windows Update. If your PC fails genuine validation, you may occasionly receive a message reminding you that "Windows might not be genuine". Besides, desktop wallpaper will change to a plain black background. You may reset desktop background to a favorite wallpaper, but it will reset to this plain black background every 60 minutes.
For more detail, you may refer to Microsoft official web page: