Free antivirus?

If you're on Win 10, just use built in Windows Security.

EDIT: I see you updated your post to say you're staying on Win 7 - that is out of support, so... Why wait to go back to Linux?

Man... Windows defender by default and some addblocks on the browser... And good comon sence browsing...
You don't need anything else

I use Spywareblaster too. You install one time, run it, and forget about it. Other than that. Common sence is the best antivirus

Avast AntiVirus is good in my opinion.

Avira is crap. Promote all the time stuff, in the paid version too!. The paid version don't found half of the stuff that the free Avast found at my dad's system.
Since i switched i never looked back because Mcaffee was crap, Norton was good to use but really horrible at the deinstall and i don't found a GOOD one besides Avast (after 1 year of free use i changed to pay for it).

And i use always on top, only started when needed, Malwarebytes Anti-Malware because it is a very good one against Malware, spyware and so on (but no antivirus)

How about Kaspersky?

Note: sorry to tell you this... If you are staying with win7 even though support to it is over... You are just being kinda... Just update to win10... There is no advantage on having win7 unless you have a really old potato...

“Free antivirus” is an oxymoron.

I don't even use an antivirus. Just a firewall and WinPatrol. And common sense.

As someone else pointed out, Windows Defender is good enough, if you feel you must run that kind of thing.

You can also use a sandbox application to run dubious apps and sites. When you're done, just clean the box.
I use sandboxie for that. Also good for install and test programs inside the box.

Comodo Antivirus + Comodo Firewall ?

Windows Security was proven (By Av-Test) to be more effective than any other free antivirus and it's free as is included with the OS, soooo...

I use the built-in Windows anti-virus, exercise a little care, and supplement with SuperAntiSpyware and MalwareBytes. I've actually got a paid version of MBAM, which takes care of things on its own, but it's easy enough to run a scan or two each week manually.

I also prefer Windows 7, but it's time to update. You'll get used to it in no time, and the upgrade will probably run for free even though it's not supposed to be free any longer.

You can get Microsoft Security Essentials as a Win7 update, but I don't think it 's good as Windows Defender in Windows 10.

The free version of BitDefender anti-virus is fairly good if you only want an anti-virus, and nothing else. It has real-time and on-demand scanning for viruses and that's it. It does eat a lot of RAM though, up to 0.5GB when idle.

Consider getting the free version of Malwarebytes Anti-Malware as well. You don't need to have it running all the time if you don't want to, but it only needs ~50MB RAM. It isn't an anti-virus though, it's meant to detect malicious programs like keyloggers or spyware rather than viruses.

Consider the motivations of each company here. The antivirus companies want to profit off of you and they know that a lot of people won't be buying the full version. So they look for other ways to make money. Maybe by making the free version useless, maybe by selling your data, maybe by some other scummy behaviour. It's simple business. They will sell what the customers are buying and, since you're not paying money, you're not the customer here.

Now consider Microsoft. You became a customer of theirs when you bought Windows. They already have your money and, because they control OS updates, they don't need you to use their AV for them to get access to your data. So they have no reason to do anything dodgy with the AV.

I have Microsoft Security Essentials on my Win7 box and using Windows Defender on my Win10 box and haven't run into any issues for years now (knock on wood). If I'm going to go to a site I think (or pretty much know) may be questionable, I'll boot up my Linux box or use a VM for that little bit.

As others have mentioned, Windows Defender on Win 10 is actually pretty good. If you are determined to stay with Win 7 (and I absolutely sympathize, I put off updating to Win 10 for as long as possible), then that OS's built in security program isn't quite as good but it is still going to catch most everything. Honestly, for ordinary internet use that might even be enough and riskier online behavior needs something stronger than a mere anti-virus program (i.e. a sandbox).

I usually have Malwarebytes installed as a secondary anti-virus. It was always the best at removing any malware once it had been installed and works well with any other anti-virus you want to have installed (e.g. Windows Defender). Malwarebytes is nice also since it does a good job with things like spyware which are far more common these days than a traditional virus. It takes up very little RAM and plays well with every other anti-virus program of which I am aware. I would advise getting it regardless of what anti-virus you end up downloading.

It is by design that the task tray icon of Avira becomes "dumb".
There seems to be no problem with security.
But there is a cure.
Select Avira service from task tray and restart to improve.

Service name? wait....... Avira.ServiceHost< This!

I only recommend two: Bitdefender and Norton Antivirus...used them both since the early 2002s and they've never let me down...unlike all the others. Some are just not intelligent enough like cough cough not naming any names, some are just bloatware and others are just plain intrusive and will stalk you after your sub expires (looking at you Panda)

Norton and Bitdefender regularly give out free 3 month trials so you have plenty of time to try it for yourself. Haven't paid for a Bitdefender sub in almost 2 years.
https://www.bitdefender.com/media/html/consumer/new/get-your-90-day-trial-opt/index.html

You don't need a second antivirus as these programs are not made to work together. Having more than one is just setting up yourself for un-necessary conflict.
Lastly whatever you do...DO NOT rely on Windows Defender...unless you have nothing of importance on your computer. It does not have good intrusion protection...if any at all. and offers only the bare minimum in terms of protection.

Check with your ISP, they often offer free Antivirus programs also. Like how Xfinity / Comcast offers Norton for free.

I'm using Win 7 and personally use AVG with Silent Mode and Games Folder on "Exceptions"

Malwarebytes and Sophos.

Windows defender and a bit of caution have been good enough for me. Although it doesn't hurt to bring several other companions along for the job.

Out-of-trial MalwareBytes antivirus software is a good and reliable choice - it even lets you download the latest updates despite not purchasing a subscription. One thing - only the purchased version has the automatic features available for use, so you'll have to manually schedule the scans yourself if your version is out-of-trial. It's not a hassle for me, but it may be for others. If you need a second layer of anti-virus software to check what MalwareBytes doesn't, then HitmanPro is a decent option. Both of these softwares are available for free and I've found them to hardly be taxing on your hard drive.

Norton is a decent antivirus software program, but imo it's not one of the best options due to the amount of bloatware it puts up. I admit I'm not up to date to its current version, but when I used Norton back between 2010-2015 it was a huge resource hog for my Windows 7 and Windows Vista PCs. Granted, I didn't have great specs, but I had to especially locate and eliminate any resource hogs on my system in order to have my computer run efficiently. Norton was one of them. Norton was not only a pain to uninstall, because it left remnants of itself, but it also had programs, files, and other unused code lurking around. One of the most noteworthy executable files was SymSilent.exe. SymSilent.exe was one of those leftovers that appeared before and after uninstallation. Back then, we didn't really know what SymSilent did, and I honestly still don't know what it does. What I do know is that it was kept on my computer, it can do something, but its exact purpose is unknown. Besides that, malware was known to disguise itself as SymSilent so as to remain on your computer and work around Norton. With all that considered, you can see why I'm not much of a fan of Norton.

MalwareBytes and AVG are the only two I have experience with. They are fine. You can install both and hope one catches something the other doesn't.

Comodo Internet Security suite (with HIPS and Auto-Containment turned off).

I didn't have any problems for over 10 years using it, both with WIn7 and Win10. No hassle, very few false positives.