Can enter a computer secretly and quietly and can be destructive in nature. A Trojan typically installs a virus or worm on to the computer it has entered. Trojans can also install programs that allow someone else to have control over the user’s computer. Trojans usually come attached to another file, such as an .avi, or .exe, or even a .jpg. Computer users fail to see full file extensions, so what may appear, as in reality could be Once the computer user opens up the file, the Trojan will activate.