Almost every antivirus scans files on access to ensure malware can't launch, and it also scans the entire system on demand or on a schedule you set. Once cleaning and scheduling are complete, blocking all access to malware-hosting URLs is another effective way to prevent trouble. Many extend that protection to steer users away from phishing sites, which are fraudulent websites that attempt to steal login credentials for financial and other sensitive sites. A few rate links in search results, flagging any dangerous or iffy ones.

Behavior-based detection, a feature of some antiviruses, is a two-edged sword. On the one hand, it can detect viruses that have never been seen before. On the other hand, if it's not done right, it can baffle the user with messages about perfectly legitimate programs.

Any antivirus software should eliminate spyware and other types of malware, but some also include features specifically designed for spyware protection. Features like encryption to protect sensitive data and webcam control to prevent remote spying are available in commercial editions, not free ones. However, some free editions include features like a secure deletion file shredder or a free password manager.

Installing all security updates for Windows, your web browsers, and other popular applications is one easy way to keep your PC protected. Windows 11 makes it easier than ever to stay up to date, but there are still numerous security holes in older Windows versions, popular apps, and add-ons. Scanning for vulnerabilities in the form of missing updates is a feature most often found in commercial antivirus tools, but it can also be found in some free ones. In the list below, you can see which programs include these useful features.

Editors’ Note: Given that the US government has banned new sales of Kaspersky security products, we no longer recommend them.

Around the world, researchers at independent antivirus testing labs spend their days testing antivirus tools. Some of these labs regularly release public reports on their findings. We follow Windows antivirus test results from five labs: AV-Comparatives, AVLab Cybersecurity Foundation, MRG-Effitas, SE Labs, and AV-Test Institute. AV-Test and AV-Comparatives also test Mac-specific antivirus apps.

Security companies typically pay for the privilege of being included in testing. In return, the labs supply detailed reports that can help improve their antiviruses. The number of labs that include a particular vendor is a measure of significance. In each case, the lab considered the program important enough to test, and the vendor felt the price was worthwhile. The labs don't necessarily test a vendor's free edition, but most vendors pack full protection into the free offering, enhancing premium versions with additional features.

Maybe, but probably not. To ensure that there is no malware or other threats present, antivirus software scans files and programs on your computer. The longer the time frame between each scan, the more files they will need to check. These more intensive scans may have some noticeable effects on your computer speed. Therefore it's recommended that you do more frequent scans or even run them at night so it doesn't affect your computer usage.

While antivirus software can have some impact on the speed of your computer, it's most likely not the only thing—or even the most prominent. Other factors such as unnecessary startup and background programs, insufficient RAM, or even undiagnosed malware can have more drastic effects on your computer speed than a simple antivirus scan.