The anti-virus I use the Windows defender/Microsoft security essentials. Why? because it is free and a very good anti-virus. And it would be point less to waste money on an anti-virus.

I use Avast anti-virus. It's worked really well for me, I haven't gotten any viruses since I got it, so that certainly is a good thing.

I use Windows Defender for security. If you think about it, you can never really tell how effective your anti-virus software is. It's facing luck until you get a virus. That's why I try to stay away from fishy sites.

I use avast! because it's the one that my family uses, and it's protected me with 100% accuracy so far. I don't think I've ever gotten a virus yet, not even from bad torrents, because avast! always stopped it from unpacking itself and executing. I almost feel like paying for the full version, but the extra features that it has aren't really especially appealing to me.

I use Eset smart security, it costs money, but it works and that is all I care about. I run a scan on it a few times every week to check and make sure there are no threats on my computer and if there is then they become quarantined and I don't have to worry about them Ever since I first got Eset, I haven't had a virus on my computer, so that is a good sign that it is working. It may cost money, but the safety of my computer is worth it. I would recommend this to those in need of an anti virus program. 

I use Avast! because it offers a free version, and free is about the highest price tag that I can afford. I don't really use any sites that I don't know I can trust. In fact I'd say 99% of my internet time is spent on Vizzed, I mean, why not? I can play better games on Vizzed than I can on any other site, I can talk to people on their profiles or via private messages ( = Facebook is obsolete for me), I can chat safely and in a clean environment ( = Any chat room off of Vizzed is obsolete to me), I can read and submit game reviews, walkthroughs, help threads, comments, and FAQ's on Vizzed ( = IGN, Game FAQs, and pretty much every other game themed site is obsolete for me), and I can post in forums on Vizzed, also a clean and safe environment ( = every other forum is obsolete for me), so... why would I hit another site anyway?

I use Avast and Malwarebytes.  For those who use Microsoft Security Essentials, I strongly recommend you switch to something else.

Avast is awesome, and available for free.  I have been recommending it to friends and clients.  With some adjustments in the settings, like enabling scheduled scans for both Quick and Full, as well as enabling PUP detection, it does really good.  I also make some other adjustments, but those are key.

Malwarebytes is awesome as well.  There has been many times where a main antivirus failed to detect something, ant MBAM was able to pick up the slack.  MBAM is also one of the best scanners to pick up those pesky PUPs.  No matter what antivirus is being used, MBAM is always good to have, and do a full scan at least once a month.

Microsoft Security Essentials is really bad in protection, according to many test results from antivirus testing companies.  Microsoft themselves actually recommend users to use a third-party antivirus.  Microsoft is very basic, and is actually the weakest antivirus I've seen tested.
http://goo.gl/MCQCgy

Sorry for the long unnecessary reply, force of habit on this topic.