Unfortunately, there is no such thing as "completely secure", unless of course, you unplug your wifi or keep your PC switched off. Even if you install an antimalware program free or paid version, there is always a chance that a malicious program or cyber crooks find a way to your system. I believe that apart from using the built-in Windows Defender, which is supposed to be a very good detector and remover actually, it is advisable to use another one like the free or paid version of Malwarebytes, but you can use any reputable and trustworthy AV software. Still, it is important to mention that you need to keep all your programs updated at all times to make sure that cyber criminals cannot exploit outdated bugs to infect you or gain access to your system. Another preventive measure you can apply is safe surfing, i.e., stop visiting suspicious websites (torrent, freeware, online gaming, etc) and stop clicking on third-party banner and pop-up ads, and do not open suspicious emails either. You must also be careful with "must-see" video and image links on social media. If you stick to these recommendations, there is a good chance you can keep your PC clean.