Win10 Defender does a fine job, just be smart about what you're downloading. Enable file extensions so you can tell if that 1080p film you downloaded is a 34kb EXE file, in which case you should probably cut/paste it to a USB stick, make an autorun file that points at that file and leave it on a dickhead neighbour's doorstep.
Assuming that the dickhead neighbor still runs an ancient version of Windows where autorunning any kind of .EXE file was possible. I believe they've nerfed the autorun.inf hard starting with Windows XP SP3.
Speaking of old people and computers, a good idea is to make sure that they use it with a standard account, so that most of the viruses get stopped by UAC, since I'm sure there are ones that will bypass it anyway. Of course Windows Defender is a good choice, since it's built in to the system and does a good job. You can also add Nano Adblocker and Nano Defender to the browser, with additional blocking lists for blocking ads, scams, viruses and others. Why those? Basically, Nano Adblocker is uBlock Origin on steroids, and it integrates better with Nano Defender, which blocks anti-adblockers.
I might as well recommend some other software.
SpeedCrunch is a good calculator alternative, as it's based around the keyboard, and not around some silly calculator GUI, which allows for quicker calculations. Also it has plenty of advanced features, for doing more complicated calculations.
NirSoft has plenty of small useful programs, for all kinds of tasks. Plenty of network tools, as well as tools for license key recoveries, password recoveries, system tweaking and more.
NirLauncher allows for downloading and accessing all of them in one place.
Sysinternals is a great set of sysadmin tools from Microsoft, which also includes Process Explorer, which is a Process Manager on serious 'roids. It also has tools for managing autorun, monitoring programs or causing blue screens, because reasons. It can also be added to NirLauncher.
Soulseek is one of those P2P programs from the Napster era, with one major difference. It didn't died out and it's still active, offering a large library of user shared files, mainly music, including very rare releases that you wouldn't be able to find anywhere else.
JDownloader 2 is a very useful download manager, which looks like it will turn your computer into something from the Exorcist, but it won't, as long as you download it from the right source (linked here). It's great for downloading from those sketchy sites with fake download buttons, waiting times and captchas, as it will automatically process the links, and will (partly) automatically solve the captchas for you. Also good for downloading YouTube videos, large files and doing batch downloading.
Dark theme is available here, although it requires a reinstall every update, and also disabling everything in advanced setting in results for "Premium Alert", "Special Deal", "Donate" and "Banner" is highly recommended for ad removal.
WinAero Tweaker is a very useful utility for tweaking Windows. It's one of my mandatory programs for every fresh Windows install, as vanilla Windows is garbage and always needs some tweaking and personalization.