Just as the mythological Trojan Horse appeared to be a gift, but turned out to contain Greek soldiers who overtook the city of Troy, today's Trojan Horses are computer programs that appear to be useful software, but instead they compromise your security and cause a lot of damage. A recent Trojan Horse came in the form of an e-mail that included attachments claiming to be Microsoft security updates, but turned out to be viruses that attempted to disable antivirus and firewall software.

Trojan Horse (n.) A computer program that appears to be useful but that actually does damage.

Trojan Horses spread when people are lured into opening a program because they think it comes from a legitimate source. To better protect users, Microsoft often sends out security bulletins via e-mail, but they will never contain attachments. Microsoft also publishes security alerts on their Security Web site before they e-mail them to the customer.

Trojan Horses can also be included in software that you download for free. Never download software from a source that you don't trust. Always download Microsoft updates and patches from Microsoft Windows Update or Microsoft Office Update. 

Be very careful when downloading from P2P software like Kazaa, Morpheus or Win-MX. What may appear as a good and proper download may indeed be a virus or Trojan Horse in disguise.