Trojans found on computers were named for the famous wooden Trojan horse that the Greek army gave to the city of Troy as a peace offering during the Greek and Trojan war. The people of Troy took the horse inside the city walls. Greek soldiers were hiding in the belly of the horse and emerged in the middle of the night to slaughter their enemy.

Today’s Trojans are any computer program that intentionally hides itself by pretending to be something else so it can perform malicious actions on your computer. Unlike their malicious counterparts, viruses and worms, Trojans can not act autonomously and spread from machine to machine. They require human intervention to activate and spread them. That means you have to open the file they are hiding in before they can do any harm.

Another difference between viruses, worms and Trojans is that Trojans do not make copies of themselves on the infected computer. They must be attached to another file and can only be spread by sharing that file.

Trojans can be very destructive and have also become a source of a new kind of criminal activity. According to USA Today, the FBI estimate of U.S. businesses losses to computer-related crimes is approximately $67.2 billion annually. Consumer Reports estimates that U.S. consumers lost $8 billion because of viruses, spyware and Trojans, especially the Trojans used for identity theft and phising scams.

Trojans are dangerous beasts and capable of doing a lot of damage such as:

• Uploading or downloading files
• Erasing or overwriting the data on your hard drive
• Encrypting your entire hard drive and then demanding a ransom to unencrypt it. This is a fairly new type of crime.
• Sending email to your entire address book which is usually some form of malware or spam.
• Creating a backdoor for someone else to take control of your computer
• Recording your general activities using screen shots and sending the information to their masters
• Recording keystrokes to steal credit card or password information to be used for identity theft.

Getting Infected

Trojans usually enter a computer systems in one of two ways. The first is through legitimate software that someone has deliberately corrupted with malicious code. The second is through an independent program that is embedded in a game, image, or song. Typically they are aimed at Windows programs and bear filename extensions such as .exe, .com, .bat, .scr, .vbs or .pif..

An actual Trojan file sent out through email was named LOVE-LETTER-FOR-YOU.TXT.vbs. Notice that the file has two extensions: .txt and .vbs. Only the last one matters because that is how your computer sees the file and it is an executable file capable of running the Trojans program.

Trojans can infect you computer in several different ways such as:

• Being attached to an instant message
• Delivered on CD-ROM
• Downloaded from websites or file-sharing programs
• As an email attachment which is the most common method for spreading.
• Hidden in games (the second most common method), utilities and other programs

Protecting Yourself

Here are some guidelines for protecting yourself from Trojans:

• Purchase a program specifically designed to detect and remove Trojans and worms as well as viruses. Configure the application to periodically check for new threat updates.
• Do not download files from anywhere without scanning them first. Make sure the application you bought scans all download files and all incoming email messages.
• Unhide your file extensions by opening windows explorer, selecting Tools -> Folder Options then click on the Files tab and uncheck Hide extensions for know file types. That way you can tell that mynewbaby.jpg is really a picture of your friend’s new baby and not a file named mynewbaby.jpg.exe containing a Trojan.
• Configure your firewall to block Trojans.

Removing Trojans

Anti Virus Programs - Some anti virus programs can detect and remove Trojans. However, they usually can not detect all of them. Make certain that you always have the latest updates. Configure the program to automatically check its parent website for new updates.

Anti Trojan Software - these programs are specifically designed to detect and remove Trojans. Most commercial versions cost around $30 and are well worth the price.

Clean Re-Install - this is the most draconian method but, if all else fails, reformatting the hard drive and reinstalling everything from the original CDs will guarantee you get rid of all viruses, spyware and Trojans.

If you want to salvage file you created, scan them with both the anti virus and anti Trojan programs to be certain they are not infected before you restore them to your machine.