Computer viruses are programs that contain the software instructions necessary to make exact replicas of themselves and insert these instructions into other executable programs. Each time an infected program is run, the virus code is executed, usually resulting in the infection of other programs. Computer viruses usually replace the first few instructions in the host program, so that the virus can run first. The virus continues to look for uninfected files each time it is run, and may eventually spread to many of your programs.
Computer viruses vary in the degree of harm that they can cause.
Some display a humorous or innocuous message, while others are designed to cause catastrophic damage. Even seemingly innocuous viruses can inadvertently cause problems or unpredictable behavior with your programs, just by being present in the System folder or on the hard drive. For example, if the virus is using memory or disk resources that another program needs, that program could crash. As the name implies, a trojan horse masquerades as a legitimate program, but in reality harbors code that could inflict serious damage to your computer. Trojanised programs cannot be disinfected and therefore must be deleted.