There may be significantly fewer viruses now than in the days of Windows XP, but they still exist today, thanks to zero-day vulnerabilities and users who get tricked into clicking or installing something that infects them. An antivirus can still be useful to protect against popular viruses that have spread on other computers. The antivirus companies have learned about them and have updated their own databases to stop those viruses from infecting other PCs.
There are several free, useful tools that do a decent job of protecting PC users from viruses. Each one protects the PC in a slightly different way, which makes them work well together.
Until Windows 8 was released, I would have recommended Microsoft Security Essentials antivirus because it used to be one of the best free antiviruses out there, and it was also the easiest to use (the same "set it and forget it" principle). But after it started coming preinstalled in Windows 8, I've noticed that it has become much less effective at catching viruses, both from sites that benchmark the effectiveness of antiviruses as well as from personal experience. (Avira would catch malware that Windows Defender wouldn't, for example.)
Avira also used to be more annoying, with an almost-full-screen pop-up ad that appeared once a day. However, these days, the pop-up appears only in a corner, and is much smaller and less obtrusive, so it hardly bothers me at all.
Germany-based Avira also has been consistently at the top of the rankings for both free and paid antivirus software. It may not be the very best out there, but for a free antivirus, it's good enough. Much like the old MSE, you don't have to tinker with it, and you almost forget it exists on your PC until it catches some malware and alerts you to it. Unlike most other antiviruses, it's also quite lightweight in terms of resource consumption.
Avira also has some nice extra features, such as automatic blocking of "autorun" executables from external media devices (such as a DVD or USB), so it should protect you from automatically getting infected when inserting someone else's DVDs or USBs into your PC. It also protects against malicious changes to Windows' "hosts" file.
There are viruses and other types of malware — such as ransomware, Trojans, spyware, adware, and exploits — that an antivirus might not catch. Malwarebytes is generally the most recommended free tool to help catch the type of malware that your antivirus might ignore.
Malwarebytes does have one major weakness: It doesn't automatically scan or catch such malware in the free version; you have to scan manually every time to check your PC for malware. Scanning about once a week should do the trick, though. In case something got through the other defenses, at least it won't have very long to do much damage.