Antivirus software (or anti-virus software), if properly installed on a computer system, it can prevent access to computer systems by unwanted computer malwares. Viruses, worms or Trojan Horses can be used by criminals or mischievous people (called 'hackers'). They can be used to steal information or damage system files. If no antivirus software is installed, hackers may be able to access the information in the computer.