If your computer has a different version of Skype installed ( not the one that comes built-in on Windows 10) then you can easily prevent it from starting from the task manager. This is the best way to stop Skype from starting automatically when booting Windows 10. This will allow you to sign out from the app. This is found on the upper left side of the app. It should appear when you click on show hidden icons. You can do this by clicking on its icon from the taskbar.
