To detect threats, antivirus products today don't rely solely on the traditional signature files (regularly updated files that identify the latest malware). They also use heuristic techniques to detect and block infections that no one has seen yet. Given how frequently new viruses crop up in the wild, the ability to protect against unknown malware is critical.
Not sure what to use? Consult our latest reviews of antivirus software and security suites. And you don't even have to spend money to protect yourself, thanks to various free antivirus options.