A trojan horse is a destructive program that will be like an application.It does not create copies of itself like viruses.Viruses usually create copies of itself by targeting some files in the computer usually exe files , but a trojan horse make a computer easily hackable in a network.There were many trojan horses , we TipsnTricks4All found was Beast and Play mp3.exe  , they make your computer easily hackable when your computer is connected to the internet , it steals data , keys and do more destructions , as it can make your computer easily hackable , the hackers can therefore access your system as you access it , they can do anything and ofcourse everything , they can trap what you type, catch your screen,steal your data,install and uninstall softwares in your computer,download and upload files to and from your computer and so on.When you doubt that someone is using your computer or your internet  this usually observed when your internet connection is slow than what your broadband or internet plan is , for example if your broadband plan is at a speed of 1 mbps and is at speed of 5 kbps or so , you better be careful and scan your computer with your anti virus.For best anti virus suggestions click here