By default, Microsoft Teams is set to automatically launch when you log in. This setting applies to your current user profile on the computer. To prevent Teams from starting automatically, follow these steps:

1. Open the Microsoft Teams application.
2. Click on three dots (Settings and more) next to your profile picture in the top right corner.
   
3. Select Settings from the dropdown menu.
4. In the Settings window, navigate to the General tab.
5. Locate the option labeled Auto-start Teams and uncheck the box  to have it not start or check the box to have it start automatically.
   
6. Close the Settings window.