If you're on Windows 7, Microsoft Security Essentials or Avast. If you're on Windows 8, it comes with Windows Defender, which is basically Microsoft Security Essentials.
Right now I have AVG on my desktop, and I would recommend it, but it appears they no longer offer the free version. I still get updates, though, because I started using it before the free version stopped being offered.
I would also recommend Microsoft Security Essentials. Norton is ok also. But honestly it's not really virus that people get today but spyware. So I would actually recommend the MSE along with a paid version of Malwarebytes.
In Windows I always had Kaspersky (Good Russian quality). Since a few years I use Sophos on MacOS and Windows because I use Sophos UTM
This is a great combination! It stops everything if you set it up right.
I also recommend Malwarebytes, I use the free version. You have to run it manually every now and then.
We roll out ESET for our business clients - the centralized management console is very good and detection rates have proven to be excellent. It also has a very nice linux based boot image you can use to clean up an infected PC without actually booting into windows. It has teamviewer built in, so you can remote in if you can manage to instruct someone on how to boot into it. Great for far-away clients/friends/relatives needing help. I've also been recommending the standalone ESET NOD32 Antivirus to individual users and have had no complaints so far. I don't ever go for the "Super Deluxe Security Suite" packages for any brand antivirus product, as those seem to bog down a computer better than most infections.
Avoid Symantec/Norton and McAfee products. I've had poor luck with free antivirus solutions such as AVG and TrendMicro in both management and virus detection capability. Microsoft Security Essentials / Windows Defender has gone down in quality significantly in the past couple of years. Microsoft even admitted that their stuff was to be considered "baseline".
I've heard very good things about BitDefender - and we will probably begin pushing that to some of our clients in the next few months.
My recommended stack of protection software to folks running windows is as follows:
Internet Protection - uBlock (yes, adblock is a form of protection - this is available for Firefox, Chrome, and Safari)
Antivirus Protection - Malware Bytes Premium
ESET NOD32 Antivirus
Slightly veering off topic:
If you're wanting protection against file encryption malware, your best best is to treat an infection as a disk failure and be sure that you have implemented a solid backup system. Cloud based services such as IDrive, Mozy, Crashplan, and Backblaze are good and automated so you don't have to remember to plug a USB drive in.
ESET Nod32 and I swear by it.
Not bothered by the review mentioned about memory usage.
I don't get slow downs or anything like it. Don't have infections and it's by far the best.
I've used it easy 8-9 years and I cannot tell you the last time I had a virus.
The amount of PC's from clients (outside of forums stuff I mean now) which I've had to repair because they use cheap, or free antivirus software, is simply unreal.
Anyone using FREE AVG or anything like that, may as well use the PC without any protection whatsoever.