These days, the Internet predators are more dangerous than ever. So dangerous, that at times, even the top notch AV/IS suites can't catch them all.
I want to stress the issue of using your built in virus protection (Windows Defender) on a daily basis, not just when it's not in use one afternoon per month. WD has settings to control scanning options, it should be set to do a full scan on a daily basis. This can be done at a time of day when you know that you're going to be using the computer.
Also, the amount of CPU usage can be changed to suit each users needs. By default, it's 50%, but if you need more of your CPU cycles for your usage, this can be adjusted lower. Or higher, if all one is doing is web browsing.
My point is "just do it". Scan your computer daily to find & quarantine threats before they cause severe damage to your OS & privacy.
It's good to have a more powerful malware scanner on hand to catch the real nasty stuff that many AV's can't touch. Malwarebytes (MBAM) is one of those options, & the #1 choice for many users. Once a week, update it & run a full scan with it, if it's there, usually MBAM will find it. MBAM (free version) can be obtained here.
Download Malwarebytes Anti-Malware 1.61 - FileHippo.com
Usually, upon a new install, a 14 day trial of the active version will be offered, if so, accept, the active version blocks access to many known malware laden sites. I run the Lifetime Pro version on 4 Windows installs, it works.
Finally, there is one last tool that I'd like to promote. It's potent, finds many things that many AV/IS suites misses, & is a great tool to run at least once a month. The ESET Online Scanner. It runs best when using IE, as there are more scanning options to choose from. Get it here, for free.
ESET :: Get a FREE Online Virus Scan
These are all great tools, but not if they're not ran on a regular basis. These tools provides with most everything you need to keep your computer virus & malware free, at no cost to you.
And keep in mind, that virus scanning is not an "optional" thing to do, but a maintenance item, such as defrag & backup. Your computer is a machine, just as your ride is. Your ride needs brake pads, alignments, tune ups, oil changes & plenty more. Keeping up the maintenance on a computer costs a fraction of what car/other ride does.
Not to mention virus cleaning's main benefit. You'll have a more secure, safe, faster & overall more pleasant computing experience by running these scans. And peace of mind.