Antivirus software consists of computer programs that attempt to identify, thwart and eliminate computer viruses and other malicious software (malware).

Antivirus software typically uses two different techniques to accomplish this:

• Examining (scanning) files to look for known viruses matching definitions in a virus dictionary
• Identifying suspicious behavior from any computer program which might indicate infection. Such analysis may include data captures, port monitoring and other methods.

Most commercial antivirus software uses both of these approaches, with an emphasis on the virus dictionary approach.

Historically, the term antivirus has also been used for benign computer viruses that spread and combated malicious viruses. This was common on the Amiga computer platform.

What is a Computer Virus

A Computer Virus is a computer program which distributes copies of itself, even without permission or knowledge of the user. A computer virus is often simply called a virus if it is clear from the context that it does not refer to a biological virus.

The term is commonly used to refer to a range of malware, but a true virus does not need to be harmful. To distribute itself, a virus needs to be executed or interpreted. Viruses often hide themselves inside other programs to be executed.

The term comes from the term virus in biology. A computer virus reproduces by making, possibly evolved, copies of itself in the computer's memory, storage, or over a network. This is similar to the way a biological virus works.

Computer viruses can spread very fast. It is estimated that the Mydoom worm infected a quarter-million computers in a single day in January 2004. Another example is the ILOVEYOU worm, which had a similar effect in 2000.

There are many viruses operating in the general Internet today, and new ones are discovered every day.

Trojan horse

Trojan Horses are impostor files that claim to be something desirable but, in fact, are malicious. Rather than insert code into existing files, a Trojan horse appears to do one thing when in fact it does something entirely different, and potentially malicious, such as erase files. Trojans can also open back doors so that computer hackers can gain access to passwords and other personal information stored on a computer.

Although often referred to as such, Trojan horses are not viruses in the strict sense because they cannot replicate automatically. For a Trojan horse to spread, it must be invited onto a computer by the user opening an email attachment or downloading and running a file from the Internet, for example.

File-Destructive Trojans

The only function of these Trojans is to destroy and delete files. They can automatically delete all the core system files on your machine. The Trojan could be controlled by the attacker or could be programmed to strike like a logic bomb, starting on a specific day or hour.

Denial of Service Trojans

The main idea behind Denial of Service (DoS) Attack Trojans is to generate a lot of Internet traffic on the victim’s machine, to the extent that the Internet connection is too overloaded to let the user visit a website or download anything. Another variation of a DoS Trojan is the mail-bomb Trojan, whose main aim is to infect as many machines as possible and simultaneously attack specific email addresses with random subjects and contents that cannot be filtered.

These will also slow your computer down greatly, not allowing you to do virtually anything, you will be able to move your mouse cursor across the screen but not be able to open any application software/mp3 files, eventually if the code is not built to be malicious then it will start to exit the number of website page's/Internet explorer applications that it has opened, giving the user access to the computer again.

Proxy/Wingate Trojans

These types of Trojan turn the victim’s computer into a proxy/wingate server. That way, the infected computer is available to the whole world to be used for anonymous access to various risky Internet services. The attacker can register domains, commit fraud with stolen credit cards or do other illegal activities without being traced.

FTP Trojans

These trojans are probably the most simple. File Transfer Protocol typically allows anonymous access to your machine, much like a web server would. It was once commonly asked that anonymous users provide their e-mail address as a password for anonymous access. The typical FTP provision is to allow anonymous users access only to your /pub directory. Password-protection is used to allow access to writing files, although some administrators feel comfortable with allowing such access to anonymous users.

This distinguishes the trojan from the FTP utility: attackers can connect to your computer with unlimited access. This protocol has been around under UNIX for decades, and it's among reasons why protected file systems often require explicit permission for each type of possible access to a file or device (read, write, expand, rename|destroy, execute, change permission, and who or what). Open properties on a file in the window of an FTP directory Universal Resource Locator to see what this means.

Worm

A worm is a piece of software that uses computer networks and security flaws to create copies of itself. A copy of the worm will scan the network for any other machine that has a specific security flaw. It replicates itself to the new machine using the security flaw, and then begins scanning and replicating a new worm.

Worms are programs that replicate themselves from system to system without the use of a host file. This is in contrast to viruses, which requires the spreading of an infected host file. Although worms generally exist inside of other files, often Word or Excel documents, there is a difference between how worms and viruses use the host file. Usually the worm will release a document that already has the "worm" macro inside the document. The entire document will travel from computer to computer, so the entire document should be considered the worm. Mydoom and ILOVEYOU are two examples of worms.