In the process of the Troy siege, the Greeks gave a large horse made of wood to the Trojans who thought it was a gift and moved the horse inside their city. Well, the horse appeared to be full of Greek soldiers, who left the horse at night to open the city gates to the Greek army. Trojan horse programs operate in the way alike. They sometimes seem to be legitimate, interesting or even useful, those able to cause severe damaged if initialized. The trojan may be distributed as a part of an application, or may install itself to the executable of normally OK programs, making itself difficult to track and combat.

Trojan horse can not self-replicate, unlike other malware applications like viruses or worms. It is usually attached to some program, or is distributed under the mask of a useful application. The trojan program may perform certain useful functions, which are however serve as a camouflage for some different hidden activity. Trojan spy function is not rare. While active, this part of the harmful code allows the third party to control you computer, which results in your PC becoming the so called "zombie computer". "Zombie computers" are used to build networks, which are later used for spamming or launching DDoS attacks.

Trojan horses differ from the computer viruses in general in the following ways: a Trojan horse is usually a normal software application and cannot spread itself by means of the user's computer. The Trojan horses were originally designed without the idea of self-spreading. The main idea was to fool people, making them allow the program to perform certain malicious functions, which they would never let it do otherwise. However, the latest Trojan viruses do contain the function of self-spreading, therefore making it hard to distinguish between a virus and a Trojan horse. That is why Trojans are often being treated as ordinary viruses, which can be both considered to be right or wrong.

Trojan horses can do different harmful things. For instance, they can:

- erase or substitute data on your computer's hard-disk;
- corrupt files in the invisible mode;
- spread other malicious software, sometimes viruses (the Trojan horses of this type are called 'droppers')'
- install networks of "zombie computers" to perform DDoS attacks or send bulk e-mail (spamming);
- spy on the user's activity and send the data to someone who would definitely use it in a way you don't like;
- log keystrokes to get the information about your passwords or credit card numbers;
- install a backdoor into your operating system.

Trojan remove process is very complicated. Sometimes the anti virus software is unable to track the Trojan horse. So the best way is to protect your system from being infected by a Trojan horse. If you never download, open and execute suspicious applications, documents and files, the chance of being infected by a Trojan drastically diminishes. Make sure your system is equipped with the up-to-date anti virus software and a firewall. Remember - it is much easier to protect your PC than to cure it after the infection.