This is a form of malware that hides inside a legitimate program that you can download such as free games, movies, antivirus programs. So be careful what you download because you might get something unwanted. Some of the big Trojan horse names are Sub7, Beast, Vundo, Koobface, zeus and many more. Zeus' purpose was to steal banking information with the use of keyloggers that record all key types and can easily find passwords and usernames. Zeus was also the program behind the US Department of Transportation information leak. This also affected NASA, Amazon, ABC, Bank of America and many other large companies. The best way to combat this problem is to make sure your antivirus software is active and up to date. Whatever AVG is, Avast. They will have their definitions updated continuously to combat any newly created viruses. Things have changed lately: it's less about bragging rights and more about monetary gain. Some of the means they use to do this are by creating a large network of computer bots that can be used for spamming or a DDOS (distributed denial of service) attack. They use viruses that encrypt commercial databases and use DDOS attacks and demand ransom to stop them. (Nash) Reason to write a Trojan horse. They can give a hacker remote access to your system. It can crash your computer with a blue screen of death. Reformat hard drive, delete date or steal money. One scare that has been in the news a lot lately is accessing the infected webcam and demanding a ransom to not be released publicly. The effects of a Trojan horse are almost endless with which you can hide anything you want