Despite being, along with worms, one of the most common types of malicious code, Trojans often go completely unnoticed by computer users. Yet many users who either don’t have an antivirus or simply don’t ensure it is updated could well be infected by Trojans.

Trojans are generally designed to provide access to  confidential information in computers such as IP addresses or user names and passwords. Similarly, many of them are programmed to resend this information to their creators. Unlike other malicious code, Trojans do not reproduce by infecting other machines and so, on their own, their ability to spread is limited.

They can however, cause a serious of other actions and the following classification of Trojans is centered on these actions:

- Destructive Trojans are designed to delete certain files, format the hard disk, or carry out similar destructive action. These malicious code normally  run once in each computer infected and to that extent, they are similar to other types of computer virus.

- Backdoor Trojans or remote control programs are designed to let hackers enter and take remote control of a computer. For this reason they have a client/server structure: the server installs on the machine under attack and the client is used by the hacker to control the computer.

-Keylogger Trojans are programmed to capture the user’s keystrokes. This information is stored in a special log that can be accessed by the attacker.

- Fake Trojans display false error messages prompting users to enter user names and passwords which are then sent on to the creator of malicious code. 

With these kinds of threats around, it is essential that computers are protected with antiviruses that include daily updates to the virus signature file and is able to detect these viruses. It is also increasingly important however that PC’s have firewall protection against those Trojans which often exploit unprotected communication ports. So nowadays a good antivirus needs to be able to deal with both viruses and the increased threat of hacker attacks. This is the case, for example, with Panda Antivirus Platinum 7 which includes a latest generation personal firewall and automatic updates every day to protect against all types of Internet-borne threat.