If you are still using Microsoft’s free built-in Defender, you might want to reconsider. It doesn’t have good Malware protection, it let as much as 20 percent of zero-day malware through in recent tests, getting only 3/6 marks. Avast free product gets full marks for protection and 4.5/6 for performance. Head over to https://www.av-test.org/en/antivirus/home-windows/windows-10/ for a full guide on which Anti Virus product is right for you