If you do not use Skype constantly probably do not want to Skype that works automatically every time you start your computer.
If you want to stop Skype from work automatically with Windows startup, follow these steps :
Open the Skype window and go to the Tools menu and select Options in the bottom of the list as the following picture ..
Now in the General settings , remove the selection from the front of the option: Start Skype when I start Windows, then press the Save button at the bottom see the following picture ..
Now Skype will not work automatically with Windows, you can start Skype by clicking on the program icon on the desktop or through the Start menu and select Skype.