[sudarshan]

from a computer virus?

[rhyaniski]

Trojan horse is a malicious program that is disguised as legitimate software.A Trojan horse program has a useful and desired function, or at least it has the appearance of having such. In most cases the program performs other, undesired functions, but not always. The useful, or seemingly useful, functions serve as camouflage for these undesired functions. The kind of undesired functions are not part of the definition of a Trojan Horse; they can be of any kind.In practice, Trojan Horses in the wild often contain spying functions (such as a packet sniffer) or backdoor functions that allow a computer, unbeknownst to the owner, to be remotely controlled from the network, creating a "zombie computer". Because Trojan horses often have these harmful functions, there often arises the misunderstanding that such functions define a Trojan Horse.The basic difference from computer viruses is: a Trojan horse is technically a normal computer program and does not possess the means to spread itself. Originally Trojan horses were not designed to spread themselves. They relied on fooling people to allow the program to perform actions that they would otherwise not have voluntarily performed. Trojans of recent times also contain functions and strategies that enable their spreading. This moves them closer to the definition of computer viruses, and it becomes difficult to clearly distinguish such mixed programs between Trojan horses and viruses.