Hi Ludwig,

Like other anti-malware applications, Windows Defender automatically runs in the background, scanning files when they’re accessed and before you open them.
You don’t really have to think about Windows Defender at all. It will only pop up and inform you when it finds malware. It won’t even ask you what you want to do with the malicious software it finds, it will clean it up and quarantine the files automatically. You’ll see a “Malware detected” notification saying “Windows Defender is taking action to clean detected malware” or “Detected threats are being cleaned.” It’ll appear in the notification center, too.

Antivirus definition updates will automatically arrive through Windows Update and be installed like any other system update. These types of updates don’t require rebooting your computer. You don’t need to worry about updating Windows Defender.

By default, Windows Defender automatically enables real-time protection, cloud-based protection, and sample submission. Real-time protection ensures Windows Defender automatically finds malware by scanning your system in real time. You could disable this for a short period of time if necessary for performance reasons, but Windows Defender will automatically re-enable real-time protection to keep you safe later. Cloud-based protection and sample submission allow Windows Defender to share information about threats and the actual malware files it detects with Microsoft.

Manual Scans

Scroll down to the Version info section at the bottom of the Windows Defender pane in the Settings window and click “Use Windows Defender” to access the Windows Defender desktop app interface. If you’ve used Microsoft Security Essentials before, you’ll immediately recognize this.

From this window, you can initiate a quick scan, full system scan, or a custom scan of specific folders. For example, you could connect an external hard drive to your computer and perform a Custom scan to scan that entire drive for malware.

If you don't want to use Windows Defender , then Windows 10 will automatically disable Windows Defender if you install another anti-malware program. It won’t continue performing real-time scans, so it won’t interfere with your other antivirus. Try to open the Windows Defender settings pane with another antivirus installed and you’ll find every option grayed out. Click the “Use Windows Defender” link and you’ll be informed Windows Defender has been disabled. Windows Defender will pop-up and say “This app has been turned off and isn’t monitoring your computer.”
If you uninstall the other antivirus, Windows Defender will kick into gear once again and take over, providing antivirus protection.

You can try Security Reviver which will scan your PC and identify security threats and vulnerabilities. Security Reviver can quickly and safely remove these threats and prevent them from reoccurring.

More infomration an be found at the below link:

http://www.reviverso...

I hope this helps in clarifying why you should use antivirus even if you have Windows 10.

Regards,
Lokesh