How could my computer become infected by malicious software?

Computers can be attacked in many ways: An attacker can intentionally install malicious software on your computer. Computer users can be tricked into installing malicious software themselves, believing they are installing some piece of "necessary" software. Computers can be infected with malware that actually installs other forms of malware. If a computer's security software is out of date, it could be exploited either by visiting a malicious website or by being attacked by a malicious or infected computer.