Avast Free Antivirus is an antivirus developed for Windows and Mac OS X platforms that not only detects and destroys a wide variety of known viruses, but can also detect traces of the modifications produced by unknown viruses.
It integrates fully into your operating system environment, and boasts two types of user interfaces: a simple one for casual users, and an advanced option for expert users.
It includes a resident scanner that, once activated, will remain minimized as an icon on your system tray. Each time you run a file or access an application, avast! checks it for possible threats.
The program will also keep track of and verify your backed-up data, to make sure that it won't suffer any damage in case your computer is seriously infected. The application is constantly updated to keep it current with the latest threats. And did we mention it's totally free? So what are you waiting for?