A Trojan is small, malicious program that is installed along with a more attractive one. For example, that great freeware program you got from that dodgy website? It may well be the program 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.
Trojans are not viruses, in the sense that they don't replicate or send copies of themselves to others. They are just another program that can be installed on your computer, albeit a nasty one!
What do Trojans do?
A Trojan can be very malicious indeed. Most of them are intent on controlling your PC. These are called Remote Access Trojans, or RATs for short. If someone has placed a Trojan on your computer, they'll be able to see everything that you can. Some of them can even controll your webcam. That means the attacker can see you! If you have speakers attached to the PC, they can even hear you!
If that weren't bad enough, the attacker will have access to your computer, enabling him to upload nasty things to your PC. After all, why should he store these things on his computer when he has access to yours?
Most Trojans these days, though, are placed on your computer by criminals. If you type your credit card details in to a website, for example, then the attacker can record what you type. If a criminal has controll of a lot of computer, he could also launch something called a Denial of Service attack. A DoS attack is when a lot of malicious computers attack a particular network or website. The network has so many request that it can't cope, so has to shut down. The criminals then blackmail the owner ("We'll let you have your site back if you give us money".) Many gambling sites have been hit by this type of attack.
A Trojan can also disable your security software, leaving you wide open on the internet.
How do trojans get on My Computer?
If you have an unpatched version of a Windows operating sytem (XP, ME, W98, etc) that is allowed on the internet then you have a very high chance of becoming infected. Install Service pack 2 as matter of urgency. And get all the other Windows updates. If you browse using Internet Explorer then be aware that it is notorius for being unsecure. (At one stage, it was known as the Swiss Cheese browser because it had so many holes in it!) even with all the latest patches, someone is always trying to find a new hole in Internet Explorer. If an attacker can guide you to a particular website, then your browser's security holes can be a way for them to load programmes onto your PC. A lot of people have switched to using a browser other than Internet Explorer, the most popular choice being Firefox.
But Trojans can get on to your PC many other ways: email attachments, software/music downloads, unsafe Instant Mesage clients, IRC, Peer 2 peer downloads, open ports not protected by a firewall - the list is long!
If I have Anti-Virus software, does that mean I am protected?
You are not necassirly protected from Trojans, just because you have Anti-Virus software on your computer. If you have Norton or Mcafee please remember this: Most popular does NOT mean best! (See our Anti-Virus section for more details.) The problem faced by the makers of Anti-Virus software is that Trojans are easily adapted, and new versions come out all the time. Anti-Virus software makers are constantly playing catch-up. That's why it's important to update your Anti-Virus software on a regular basis. Besides, a really good trojan can disable your Anti-Virus.
