Now it has registered about 55,000 computer viruses. Their number is constantly growing; there are brand new, previously unknown types. It is hard to classify the virus. In general, they can be divided into groups according to the following basic criteria: the habitat, the operating system and features of the algorithm works. Depending on the environment they are distinguished into file, boot and macro viruses. Initially, the most common form of computer infection was a file virus that lives in the files and folders on your computer's operating system. However, it is rather primitive viruses: they tend to very quickly find it and cannot cause an epidemic. Methods of dealing with such viruses are primitive too. Another type of computer pests is a so-called parasitic virus. They leave the infected files are completely or partially operational, but change their contents. Companion viruses do not modify the file, but create for him a double- file. The operating principle of boot viruses is based on algorithms of the operating system. These viruses infect the boot sector of floppy disks or hard drive - a special area on the disk containing the boot program on your computer. Macro viruses are kinds of computer viruses created using macro languages. Viruses can also be distinguished on the algorithms that are the various programs make them so dangerous and subtle. All viruses can be divided into resident and nonresident. Resident virus is similar to the spy, constantly working in a foreign country. Once when it loaded into the RAM on your computer, the virus remains in it until the computer is switched off or rebooted. Non-resident viruses do not infect the computer's memory and are able to breed only if they are run. To maximize the disguise of anti-virus programs, almost all viruses use methods of Self-Encrypting or polymorph. Changing its appearance (the code), viruses fully retain their ability to perform these or other malicious actions. Now there are universal methods of dealing with such viruses.