Whats a trojan and does my computer have one? - Aamod

A Trojan horse, or simply trojan, is a piece of software which appears to perform a certain action but in fact performs another, such as transmitting a computer virus. Contrary to popular belief, this action, usually encoded in a hidden payload, may or may not be actually malicious, but Trojan horses are notorious today for their use in the installation of backdoor programs. Simply put, a Trojan horse is not a computer virus. Unlike such malware, it does not propagate by self-replication but relies heavily on the exploitation of an end-user (Social engineering). It is instead a categorical attribute which can encompass many different forms of codes. Therefore, a computer worm or virus may be a Trojan horse.

Source: Wikipedia.

You can check if your computer has a trojan by using one of the popular anti-virus software like AVG Free Anti-virus.