People usually get confused between the functions of Virus and Trojans. The difference between the virus and Trojans is that a virus is a program designed to perform functions like corruption of system files instability of operating system etc. But a Trojan creates a back door or hole in the system for data thief’s or controller’s to have access to the system. This is like someone using your computer to crack into a bank. Trojans can also cause the crashing of system by accessing internet bandwidth and downloading excessive of data from the internet making the system slow and ultimately crash down. But these two can be taken care with the help of a good anti virus.