What does antivirus software do?
Antiviral software attempts to prevent, detect, and remove viral infections. There are three ways in which antiviral software attempts to do this: activity monitoring, change detection, and scanning. Most antiviral software uses or makes available all three.
The key to making sure your system is virus-free is to use antiviral software and keep it up to date. Old software is not, and cannot be, aware of new viruses.