A Trojan is a type of virus of worm that can cause serious damage to your computer. It's a program that at first glance may seem safe but in actuality contains something quite harmful. So harmful, in fact, it can ruin your computer by causing widespread damage that may be irreversible. If you remember your history, you will recall that the Greeks won the Trojan war by hiding in a large hollow wooden horse in order to enter a heavily fortified Troy without knowledge. This indeed is how a Trojan gains access to your computer. Trojans get on your computer by downloading seemingly harmless programs such as games, pictures, music or movie files, but once these programs are executed, their job begins. Trojans may only do things to annoy, but they can do worse damage. Trojans can erase your disk, send your credit card numbers and passwords to a stranger or let others use your computer for illegal purposes like denial of service attacks, crippling networks for months.

Your best protection is Anti-Virus Software that continually updates and to be careful about what you download off the internet. You only want to download from services or websites that you know to be safe.