This document introduces the importance of antivirus security systems in safeguarding computers against various types of computer viruses, which can arise from actions like downloading files and opening email attachments. It details the mechanism of different viruses, including file infectors, boot sector viruses, trojan viruses, and worms, outlining how they spread and the potential damage they can cause. Emphasizing the need for antivirus protection, the document suggests preventive measures such as regular software updates and cautious handling of email attachments to mitigate the risk of infection.