The term "Trojan Horse" is from ancient mythology. The Greek Army laid siege to the city of Troy but were unable to penetrate the strong defences. They decided to trick the enemy by building a large wooden horse and presenting it as a gift. Unbeknown to the enemy Greek soldiers were hidden inside the wooden horse. Once safe inside the enemy fortress they waited until nightfall, crept out of the horse and opened the castle gates allowing the whole of the Greek army to enter the castle and defeat the sleeping Trojan soldiers.

In computer terms there are programs that do exactly this. They enter your computer disguised as an email or free programs you have downloaded from the internet. Once safely passed your defences they deliver their payload. They may lie dormant on your computer for months before they are activated. They could give complete control of your computer to a hacker as though they were sat at your keyboard. He can read all of your files, he could access all your passwords and probably most usefully to a hacker he could launch an attack on another computer using your computer. This would mask his identity and allow him to evade detection by the computer he is attacking.

Probably the most famous trojan horse is a program called "back orifice" which is an unsubtle play on words on Microsoft's Back Office suite of programs for NT server. This program will allow anybody to have complete control over the computer or server it occupies.

To protect yourself from programs such as these you should ensure that you have a virus checker installed and it is regularly updated with the latest virus definition files. You should also use specialist software to scan your systems for existing trojan horses that may be lying dormant waiting for a hacker to discover their presence and activate them.

A personal firewall will also offer you some protection. Zone Alarm by Zone labs which is featured in our firewall tutorial will only allow authorised programs to run. So if a program on your computer tries to access the Internet you will be alerted - if you do not recognise the program do not allow it access. It should be noted however that some of the latest trojan horses have cleverly got around this by renaming themselves as explorer.exe which is Internet Explorer and this will usually already be allowed to access the Internet by the Firewall.

I have included a link below to a company called Agnitum who offer two programs that protect against the threat of Trojan Horses. These programs are called Tauscan and Jammer. Both are available to download as a 30 day trial or you can purchase them immediately. These programs offer detection and removal of existing trojans and prevent re-infection.