Malware, short for malicious software, is a type of software that becomes installed on a computer without the person using the computer knowing about it or wanting it there. There are different types of malware that can harm computers, such as viruses and spyware. Computer security software is often successful in preventing infections of malware. When security software fails or is not installed, the computer can become infected. Removing malware may be difficult, even using programs designed to remove it.