I recommend avast!. It is fast, free and effective.
Except it isn't necessarily. I found Avast to be A Vast waste of time.
It would appear that ZeroAccess can effectively render both Avast and Malwarebytes "expired" and as effective as using a blind chihuahua as a guard dog. It just sits there shivering and makes a mess on the carpet while a dozen masked men back up a truck and casually walk out with... everything... including the blind chihuahua's food dish.
While Avast scores very high in tests, so does AVG free.
If an antivirus can be disabled by telling it that it has passed its freshness date, then it isn't an antivirus at all, but more like a protection racket. I don't care what sort of grades it got in the sterile environment of a testing lab.
The thing that annoys me about AVG free is the self-promotion popups, but it is fiercely functional... Grisoft is just mightily proud of its AV and maybe rightly so, but I don't really need to be reminded constantly with popups that steal application focus at just the wrong moment.
AVG free is as good as AVG pro in most respects for most users and Pro is a really outstanding AV and scans the system from boot at install, Avast doesn't appear to do so unless you specifically run a boot scan AFTER install. I don't see how it can possibly take control of system security if something has already taken that control away from it... AVG can wrest control of an already infected system by scanning and installing at boot before most viruses have even had their morning coffee. Probably why Avast can be so easily fooled into installing as already expired... and what sort of "free" AV expires anyway?
AVG and Avast get near perfect detection marks in tests (on a clean system), but Avast is only "free" for a period of time, after which it demands an activation key and sits there in pedestrian fashion watching impassively as whatever crawls in under the door takes up residence on your HDD in precisely that way in which an actual AV program does not. When AVG Pro expires, it doesn't update definitions but it doesn't make popcorn and leave the front door ajar like Avast.
While Comodo gets only "pretty fair" marks, it does catch the really evil stuff before it can launch and it's freeware for personal use (actually free ,not pretend-free) much like AVG free. It does throw popups for paid assistance/chat/remote cleanup services, but it allows these to be shut off while just reporting when it is running a scan and it doesn't pollute your desktop with megalomaniacal random self-promotion banners (like AVG free) that cause you to type into thin air while writing that important presentation or essay, or get you killed in game because you lost application focus in the midst of a boss party.
Avast let me down severely... which is why I'm here posting in the malware log forum. So did Malwarebytes, and for the same reason.
Comodo found and killed most of the nasty bits that Zeroaccess dragged in. Nasty bits that Avast and Malwarebytes ignored because they were on strike.
So neither should ever be referred to as "free," they aren't... They are "caveatware" and subject to being disabled because of it.
When I'm done here getting the help I need mending my systems, I will be making a choice between going back to AVG (and I might even scrape up the bucks for the pro version just to remove most of the popups), or I'll stick with Comodo for a while and see how that works out. I think Comodo has an interesting strategy and it could very possibly become one of the top 5 with a little work and a little time.
One of the other criteria I use to gauge an AV is whether it uninstalls cleanly.
I haven't seen as big a mess as Avast leaves on its way out since Norton... and I wouldn't let my dog load Norton... If my dog had a computer... if I had a dog.
Norton to its credit at least has a removal tool you can download which completely removes it, cleanly, even if portions have already been removed... they hide the tool download under obscure links and force you to go on a little treasure hunt to find it, but the tool actually works very well.
Avast makes its removal tool easily found, and the tool appears to be a complete waste of time. Attempting to uninstall from Windows add/remove programs is a disaster, and the removal tool is only a little bit more effective than nothing at all once an attempt has been made to uninstall it from control panel. What a mess. I may actually have to reinstall it to remove it cleanly.
Just my two cents. YMMV.
I'm not an expert by any means but I've got a lot of miles on my odometer, and the last two weeks have all been city miles thanks in part to Avast.