For optimum protection, you should install a program that is designed to scan your system on a regular basis, so as to detect and remove malware before it has a chance to infect your system.
Some security software, such as Windows Defender, is only designed to target malicious software after your PC has already been infected – it does not defend your system against infection. Regardless of how reliable this post-infection program may be, supplement it with anti-malware that provides real-time protection 24/7.