In the not-so-distant past, legitimate fear of viruses online spurred an industry of antivirus software for PCs. A general lack of security consciousness among those making their first steps on the internet led to lost files, ruined computers, and, in the worst-case scenarios, sensitive data falling into the wrong hands. For a long time the susceptibility of Windows to viruses was a major selling point for anyone hawking Apple's wares. Now, in 2015, that relationship between platforms is a tired and inaccurate caricature. Web standards have evolved and improved, from routers, to browsers, to operating systems. The security solutions built into Windows manage to stop many threats the average user stumbles into.
That said, it can be a little perplexing that antivirus software is still a legitimate business to be in, never mind a kind of thing that actual users pay for and download. The desperation of antivirus companies became quite palpable as premium software was readily chopped up into free counterparts, if only for the chance to try and upsell nervous users. The subscription model bloomed in the face of piracy, but so too did the functionality of these antivirus programs. Feature sets expanded to offer file cleanup, protection against phishing scams, boot-up optimization, and plenty more features. Smart developers would realize they could engender themselves to users by staying out of the way with low memory footprints, rather than trying to persistently scare them into an upgrade (though many continued to do so).
Threats are still out there.
In the modern computing landscape, few users experience the historically dramatic effects of a virus, but that hardly means the threats have disappeared. Malware, which disguises itself as a legitimate application, has become a much more common occurrence. On the flashier end of the spectrum, a new breed of ransomware called CryptoLocker holds your files hostage unless you pay up. A recently-discovered Trojan, Regin, has been operating since 2008 undetected. The NSA is apparently eager to use a malware structure to extend its reach. Kaspersky alone identified over 123,000,000 unique malicious objects in 2014, and those are just the ones detected by a single service provider. Threats are still out there.
Still, many tech-savvy Windows users aren't willing to spare the system resources to run an antivirus program in the background while going about their daily routine, never mind fork over the annual subscription fee for premium products. Windows Security Essentials (or Defender, depending on which version of the OS you're running) is a common standby, primarily because it's free, provides a reliable baseline in protection, and doesn't slow down your computer. What's troubling is the confidence among users that they're visiting entirely reputable sites, and so have no need to worry about viruses. This attitude is cemented by a track record of unimpeded PC performance.
My biggest worry isn't a virus that wreaks havoc on my files or dramatically affects my system's performance while evading deletion. It's the one that that has been quietly nestled in my computer without my knowledge. After reviewing a handful of top-tier antivirus programs for Windows, at least a couple of these sleepers have been uncovered. I don't know how long they've been there, I don't know what they've recorded, and I don't know how many of my contacts they've spread to. Like most tech enthusiasts, I like to think my browsing habits are tame and I don't install anything stupid on my computer, but the fundamental truth is that there is too much going on behind the scenes for any human to monitor every little byte that drifts to and from a computer's hard drive. Though you obviously can't let yourself be paralyzed by paranoia when it comes to these things, there is a healthy degree to which you can fear the unknown.
don't confuse protection that's "good enough" with protection that's "good"
Maybe if your computer is still running in tip-top shape and all of your banking information is in order, you have no reason to care about a quiet little virus. Perhaps a few extra megabytes of RAM to play with are worth rolling the dice if the odds are in your favor. Just don't confuse protection that's "good enough" with protection that's "good". Labs like AV Comparatives, Virus Bulletin, and AV-Test that rigorously test security software put the out-of-box virus protection capabilities of Windows well behind the vast majority of commercial antivirus products out there. Even Microsoft themselves admit they offer little more than strong baseline protection (i.e. better than nothing).
Certainly don't fool yourself into thinking that you're too clever to get a virus; any free, reputable, readily available antivirus program will catch more than you'll ever be able to, even if you steer clear of shady sites and software. Being just a little extra prudent about online security doesn't just protect your own interests, either. For every virus that slips through the cracks, you act as a vector of transmission, whether you know it or not. On the other hand, the networked nature of modern antivirus software ensures a kind of herd immunity for threats as they emerge. You're not just doing yourself a favor by improving your computer's security, but you're doing a favor for everyone you connect to.