Trojan horse (sometimes called Trojan) is malicious program. Unlike viruses or worms, it is not able to replicate and infect files on its own. Most often it exists in a form of executable file with .EXE or .COM extension. Basically file itself doesn’t contain anything except malicious code. The most effective method of cleaning is very simple; deletion. Trojans can also pretend to be useful programs. This type of infiltration has various functions ranging from sending keyloggers to file deletion (e.g. to format a disc).It has also special function – installing of so called backdoor.