So what is the moral of this story? Mainly, beware of Trojan horses. But how does that relate to computers? That's a good question. In the computing world, Trojan horses are more than just a myth. They really exist and can cause damage to your computer. Trojan horses are software programs that masquerade as regular programs, such as games, disk utilities, and even antivirus programs. But if they are run, these programs can do malicious things to your computer.
For example, a Trojan horse might appear to be a computer game, but once you double-click it, the program starts writing over certain parts of your hard drive, corrupting your data. While this is certainly something you want to avoid, it is good to know that these malicious programs are only dangerous if they are given a chance to run. Also, most antivirus programs can catch Trojan horses when scanning for viruses. Unlike viruses, however, Trojan horses don't replicate themselves. Though it is possible for a Trojan horse to be attached to a virus file that spreads to multiple computers.
So as a general rule, don't open a program unless you know it is legitimate. This applies especially to e-mail attachments that are executable files. Even if you are pretty sure the attachment is OK, it is still a good idea to run it through your virus scan program (with the latest virus definitions) just to be safe. Remember what happened to the people of Troy -- don't let a Trojan horse catch you off guard.