Managing software licenses is another task that administrators don't often regard as a security measure. However, licensing is typically one of the largest IS investments. Thus, theft and piracy are major problems. As the systems administrator, you are responsible for distributing the media and preventing unauthorized copying or theft. As the systems administrator, you are also responsible for removing and preventing the installation of unauthorized applications. Installing software from unknown sources might introduce computer viruses and Trojan horses to your network. (A Trojan horse is a program that supposedly performs one task but does something very different.) Unauthorized software can produce leaks or modify system properties. Even loading network-monitoring tools can open security holes if you install the tools incorrectly. In case a virus or Trojan horse gets into your system from unauthorized software or another source, you need to carefully maintain proper permissions and ownership policies to minimize damage. Viruses and Trojan horses generally can't cause harm when you deny them access. In addition, installing antivirus software is a good idea. This software is readily available for NT. UNIX antivirus software is slowly becoming commercially available.