How to Turn on Windows Defender for Windows 10
To start Windows Defender, you have to open the Control panel and Windows Defender Settings and click on Turn On, and ensure that the following are enabled and set to On position:
1. Real-time protection
2. Cloud-based protection.
When there is no security software protecting your computer, you will see notifications like these appear.
Clicking on it will show you the installed security apps on your system. If you miss this notification, you can see it in the Notification & Action Center.
Clicking on it too will show you the installed security apps on your computer, as follows.
Select Windows Defender and then click on the Turn on button. Windows Defender will start. The first think you want to do when this happens, is to update your definitions.
Clicking on the Settings link in the top right corner will open the following panel. You can also access it via Settings > Update & security > Windows Defender.
Once here, ensure that Real-time protection and Cloud-based protection is set to On. You may also set Automatic sample submission to the On position. You may then configure Windows Defender according to your needs.