This presentation introduces computer viruses by defining them as software programs that can copy themselves and attach to other applications and files. It describes three main types of viruses: boot viruses, which infect boot records; program viruses, which infect files like .exe and .sys; and macro viruses, which infect data files like Word documents. The document explains that viruses try to take control of an infected computer system by making copies of themselves and performing potentially harmful tasks without the user's awareness.