At some point most of us have heard the term "Trojan horse" used when describing a type of virus. But many of us know little about what a Trojan horse virus actually is.

A Trojan horse virus is effectively a file or a piece of software which appears to be legitimate and serve a purpose, but is in fact harmful and capable of damaging your PC and compromising your privacy. Trojan horse viruses are also notoriously stubborn, making the virus removal process very difficult.

A Trojan horse virus gives the author of the virus complete access to a computer system remotely over the Internet. Once installed on your PC, the author can access the program immediately and use it to perform a variety of tasks. This unbridled access also allows the hacker to easily install other forms of malware including spyware, worms and other types of viruses in your system. As well as the ability to download and install these dangerous files and programs, a Trojan horse virus can crash your system and wipe your entire hard drive causing you to lose all of your important files, photographs and media files. (Often with no way of recovering them.)

Other harmful operations that can be performed by the virus author include:

Data theft. With full access to your system, the hacker can collect all of your private information including e-mail addresses and bank details.

Key-logging. Using a Trojan horse virus, the hacker can record all of the keys that are pressed on your keyboard, enabling them to collect passwords or other sensitive information as you type it.

Spamming. Often in the form of pop-up adverts, these automated spamming attacks can be carried out on your system either occasionally or in some cases continuously, preventing you from using your PC at all.

How do Trojan viruses access a PC?

There are four main origins of a Trojan horse virus.

1. Instant messaging clients.

2. Security flaws in web browsers.

3. E-mail attachments.

4. Software downloads (usually from pirated software or software downloaded from file sharing sites).

How can I tell if my system is infected with a Trojan horse virus?

• Has the operating speed of your computer slowed down considerably?

• Are you seeing unexpected messages or pop-ups?

• Does your computer keep crashing?

If you are experiencing any of the issues listed above, then it is possible that your system may be infected and that virus removal may be necessary.