Trojans 

In computers and Internet world definition of “Trojan” is commonly known as illegal program that works without users knowledge on his system. Although Trojans are at the same level of risk as viruses and worms, they are very different. Trojan isn't able to replicate itself and has certain functions. It also doesn't spread over the network or system that's why it is possible to remove Trojan using manual removal instructions, but jsut like the viruses it is hiding in the system.

Most of Trojans are fake programs. For example you download some program or a game. You install it. If that program is Trojan it will ask to restart system after installation. But in the result program doesn't work. It is said about programs .exe that are in your system but when you try to run it they just don't work. These programs are downloaded to your system without your knowledge or permission.

If Trojan doesn't run as specific program, that doesn't mean that it is harmless or useless. Trojans are able to delete some system files or files of other softwares. Also it is used for security holes. Trojans enable others to access you computer system/network. After some users have logged on your system, computer runs slower. Security holes creates free way for spyware, malware and other parasites. Hackers and crackers may take advantage of it. In that case Trojans create danger for your security. It spies your activity online and offline and even works like keylogger. It is possible that Trojan horse will bring a virus to your system, because Trojan can be infected and be like a host for virus.

As it was already said, Trojans and viruses are two different things, that's why most of anti-virus programs do not detect Trojan on your system. Much more prevention is needed to protect your PC from Trojan.

Users can prevent from Trojans. Trojans are often brought to the PC with freeware or files transmitted using P2P file sharing systems, like Kazaa. Serials, cracks, keygens that are downloaded from crackers' sites may also contain Trojans. It is also said about already cracked softwares. Surfing in sites of hacker and crackers will give you not only interesting information and ways to get free software but also a security hole in your system. Some Trojans can be attached to an e-mail message, so be aware from spams and do not open letters that are from unknown sender with unknown file. Friends should firstly warn each other before sending any .exe files.

Security can be improved by using anti-spyware and anti-trojan program. Firewall should also help. Be sure that the program has update feature and updates it's database of known Trojans. Anti-virus program should also be up to date, because most anti-virus programs collect database of all parasites, including spyware, Trojan, keyloggers, etc. If you're using Microsoft's operating system, keep it updated also, and always fix even the smallest problems. Users of Linux and other operating systems should do the same, but those systems have less problems. But if you have anti programme and don't use it, it's useless. Scan your system as often as possible.