There used to be an option in the Skype settings to disable 'run automatically on start-up.' It's gone. Like a persistent piece of malware or The Terminator, the bloody thing refuses to go away.
By default, Skype opens automatically each time your computer starts and you log in to your user account, supposedly to provide instant notifications when someone sends you a message or calls you.
Time was, you could disable Skype on startup, to manually run it when required, and it remained running with a notifier in the sys tray until you signed out or closed it.
Allegedly you can stop Skype starting automatically with Windows 10. However, the steps are apparently different depending on how you installed the program. Unluckily for me I never installed it, the bloody thing just appeared.
There are two versions of Skype can infect Windows 10: the Skype app that comes with Windows 10 and the "classic" Skype desktop app. If you obtained Windows 10 from the Microsoft Store, you can’t disable Skype on startup because there is no UI control to do so. In theory there is an option to log out of the program to close it down and prevent all notifications. That doesn't exist in any version I have either.
The Skype app in Windows 10 comes pre-installed, but the path to stop this Skype app from starting automatically is nowhere near Skype.
Open the Settings app and select Privacy, bottom-left. Scroll down to App permissions and find the Background apps tab hidden almost at the bottom.
The apps displayed on the right side of the window run in the background, even when you're not using them. Scroll down to Skype in the alphabetical list and toggle the modal button to 'Off.'
The other version is the Skype desktop app, where you have to go to... what the hell, life is too short for this Microsoft s***.
If you don't need Skype, just nuke the bastard from orbit. Go to Apps and Features, find Skype in the list and hit Uninstall.
Image credit: Terminator Endoskull - build by Kramstaar