The goal of this dissertation is making the reader acquainted with working of computer viruses and related things, together with methods applicable against them. It explains what a virus is and what is not, demonstrating them by examples. It tells the story of viruses, their evolution from the beginning to present days, their techniques to make a fool of antivirus programs, types, spreading principles. It clears up misconceptions and gives practical advices to solve and prevent problems. Beside the prosaic facts various contributions and background informations colorize the message. On the antivirus side it sets forth the early and modern procedures and operational principles used by virus scanners. It gives a survey of the virus scanners available today, compares the detection rates and points out their weaknesses and limits. The dissertation also contains a virus scanner and remover program that demonstrates explained methods in practice. Thoroughly designed structure of the program makes it possible to be a commercial grade product after an improvement. Despite it, the author intends it to be a free, open source software. All the untouched topics are examinable with the help of the carefully compiled bibliography.