Removing Trojan horse viruses from a computer is difficult or impossible to accomplish without using specialized anti-virus programs. In some cases, it may be possible to restore a computer to an earlier backup to eliminate the Trojan program if the backup itself is not affected.
Trojan horse viruses are viruses that infect computers when some other program is run. Pirated software is often infected with these viruses to allow hackers to access computers after the pirated software is run on another machine. Unfortunately, deleting the infected software does not usually remove the virus itself, as the virus introduces code into other parts of the computer and operating system separate from the infected program.
Because of the complex ways in which sophisticated Trojan horse viruses incorporate themselves into operating systems, it is very difficult or impossible to successfully remove many of these viruses manually by simply removing files or altering configurations. In most cases, a specialized virus removal tool, such as the Microsoft Windows Malicious Software Tool, must be employed to remove the virus from the computer. The best software for this task depends on the computer's operating system and the nature of the virus. In severe cases, it is sometimes simpler and more effective to reformat the computer and reinstall its operating system entirely, though this should be considered a method of last resort if effective virus removal software is available.