Trojan horse virus is one of most common but destructive program. Trojan horse virus program installed on computers usually without the owners permission or knowledge and will be present inside a computer as a hidden program or it will just act as a useful program misleading the user. A Trojan is a program that may appear to be legitimate, but in fact does something malicious.        

Named for a giant horse that was supposed to be a gift but was filled with the Greek army, a Trojan horse virus program can be just as deceptive. The story goes that the Greeks gave the Trojans a huge wooden horse as a peace offering. The citizens of Troy accepted the gift, brought the horse inside the city, threw a victory bash, then went to bed. It wasn't until the Greek soldiers had set the city on fire that they realized they'd been had. A Trojan horse that affects computers can contain some nasty surprises as well. It can damage, delete, or destroy important files.

A Trojan horse virus usually get installed on a computer along with the installation of free software. For example, that great freeware programme you got from that dodgy website? It may well be the programme you wanted. But someone (usually a 3rd party) may well have attached a Trojan to it. The Trojan will be installed as well as the software you wanted.

Once a Trojan horse virus is activated, it can access files, folders, or your entire system. Commonly, Trojans create a "backdoor" or a "trapdoor," which can be used to send your personal information to remote location. To protect your computer from a Trojan horse virus, extensive anti-virus software is a good first step. Choose a program that looks for Trojan horses and worms as well as viruses, and make sure it updates definitions for each frequently. Also, make sure your anti-virus scans email, and gives an alert or automatically deletes any message that contains suspicious code, even if the code is not specific malware that the program has already identified.