Microsoft Defender Antivirus is part of Windows Security in both Windows 11 and Windows 10. By default, Microsoft says, it runs in the background, scanning files and processes that you open or download. The same app includes Controlled folder access, which stops unauthorized apps from changing files in protected folders as a guard against ransomware, and Windows Firewall, which filters network traffic and blocks unauthorized access.
Windows 11 adds Smart App Control, which blocks apps that Microsoft's cloud security service judges malicious or potentially unwanted, and unsigned apps the service can't vouch for. Microsoft says it works alongside Defender or another antivirus, and recent updates let you turn it on in Windows Security without reinstalling Windows, if it's available for your device (in S mode it stays off). Windows 11 also needs a PC with TPM 2.0 and Secure Boot-capable firmware.
If you install another antivirus, Defender steps aside. On Windows 10 and 11 it switches to a disabled mode automatically; on Windows 11, if Smart App Control is on or in evaluation mode, Microsoft says you may see it go into passive mode instead. Microsoft also says Defender can switch itself back on if the other product expires or is uninstalled. So run one real-time antivirus, not two. To check which one is protecting you, open Windows Security, select Virus & threat protection, and under Who's protecting me? select Manage providers.
For one updated PC whose owner is careful with downloads and links, Defender already supplies the real-time antivirus layer; for how its protection compares with paid engines, check the independent labs, which test Microsoft Defender too. A suite earns its price when you'd use what it bundles: cloud backup, clean-up tools or a firewall with more controls; a VPN or a password manager; or one subscription for the family's Macs and phones too.
A note for Windows 10: Microsoft ended support on October 14, 2025, so a PC that isn't enrolled in Extended Security Updates no longer gets security updates or fixes, and Microsoft warns it's "at a greater risk for viruses and malware." It recommends upgrading to Windows 11 if the PC qualifies. If it doesn't, or you need more time, the consumer Extended Security Updates (ESU) program for PCs running Windows 10 version 22H2 delivers critical and important security updates until the program ends on October 12, 2027. An antivirus doesn't patch Windows, so treat it as an extra layer, not a fix.
