What is a computer virus?

Computer viruses:

Computer viruses are programs written by people who are up to no good, for the most part. Believe it or not some people actually use viruses to experiment. These virus programs are placed into a commonly used program and said program will run the attached virus program as it boots, therefore, it is said that the virus "infects" the executable file or program.

A virus is inactive until you execute an infected program or application, or start your computer from a disk that has infected system files. A computer virus can copy itself and infect a computer without the permission or knowledge of the owner. Once a virus is active, it loads into your computer's memory and may save itself to your hard drive or copies itself to applications or system files on disks you use.

Most personal computers are now connected to the Internet and to local area networks, facilitating the spread of malicious code. Today's viruses may also take advantage of network services such as the e-mail, World Wide Web, Instant Messaging, and file sharing systems to spread.

A virus may damage your system files. You may be able to delete all viruses from your computer or network but the damage is already done. It may cause your computer or network to run slowly, or applications may fail to start. Reinstalling the operating system may be the best route to take at times.

Trojan Horses:

A trojan horse is not a virus. Trojan horses are designed to allow a hacker remote access to a target computer system. It is a program that you run because you think it will serve a useful purpose such as a "game" or "free software". Like a "trojan horse" it serves not as it claims but facilitates unauthorized access to the user's computer system, in order to damage files or perhaps plants a virus into your computer.

A trojan horse does not replicate or spread like a virus. Most virus checking programs detect trojan horses, ideally preventing them from ever being installed.

The hacker need not be the individual responsible for distributing the Trojan horse. It is possible for hackers to scan computers on a network using a port scanner in the hope of finding one with a Trojan horse installed.