What is a Trojan Horse Virus?

I encountered this gem on the Vista Website:
The Trojan-Horse-virus!
This is an e-mail virus commonly released by an e-mail attachment and if this attached file is opened, it will scour the hard drive for any financial and personal information such as the social security, PIN numbers, account numbers. When it has collected the info, in the computer, it is transferred to the database of the thieves!


Today, there are viruses and there are Trojan Horses. However, there is no such thing as a “Trojan virus”. Actually, the real definition of each prevents the chance of the virus’ being such a thing. Trojan doesn’t replicate but Viruses do. That truth alone implies there could never be a "Trojan Horse virus".
The description of Visa continues with this message, "A Trojan Virus is an e-mail virus commonly released by an e-mail attachment." Well, not so. A Trojan could be sent with the attached file in e-mail, but it is definitely not an e-mail virus. (Actually, there are some true e-mail viruses, but that is another whole topic). Therefore, it may or may not go with e-mail, and it is equally possible to have been downloaded from websites or resulted from P2P file transfer. Therefore, vectors have nothing to do with either something is or is not a Trojan.

What is a Trojan then?
This is a common yet hard to eliminate computer threat. It is a type of virus which attempts to make the computer user think that it’s a advantageous application.
Trojan works by hiding in a set of useful software applications. Once installed or executed in your system, the virus will begin infecting other files in your computer.
The virus is as well commonly capable of stealing important info from your computer. It will then transfer this info to Internet servers assigned by the virus’ developer. The developer can then be able to get control over your PC through this virus. While these stuffs occur, you may observe that your infected computer become very slow and sometimes unexpected windows simply pop-up without any command from you. Later on, this can result your computer to crash.

The Trojan could actually spread in many ways. The most usual way of infection is by means of e-mail attachments. The virus’ developer usually utilizes different spamming tactics to spread the virus to unsuspecting users.
The best way to prevent a Trojan Horse Virus from entering and infecting your computer is to never open email attachments or files that have been sent by unidentified senders. However, not all of the files we could receive are guaranteed to be virus-free. With this, a nice way of securing your computer against malicious programs like this harmful application is to install an antivirus software and updating it.