Every Windows computer has Microsoft Defender Antivirus installed, making it the most widely installed antivirus. However, if you've installed third-party protection, Defender stays on the sidelines. If no other antivirus is present or your existing antivirus expires, Defender steps up. Commercial antivirus apps offer protection beyond what's included with Windows, but Defender is looking better lately, with some solid scores from independent testing labs. The combination of good lab scores and a great score in our hands-on malware protection test brought its rating up to 3.5 stars. As it is a free, built-in utility, however, we're not including it in this roundup of commercial antivirus apps.