Have you tried the following:
- Click on Windows Icon in your task bar.
- Type "Settings" and click the "Settings" gear icon. This should pop up a windows with the current user log in (your information)
- Look for "Apps" and click on it. A window will appear in the right side
- Type "Cortana" and click on the 3 dots, choose "Advanced option" and turn off toggle for "Run at log in".
Let me know if this helps.