I second and will expand upon an earlier poster's note about the shameful business practices of several major anti-virus makers who go out of their way to either
deliberately make their products incompatible with competitors' products, or at least claim that they are during installation. This is a crime in my view and those vendors should be prosecuted.
In my own experience, Kaspersky is the worst offender in this regard (there may be others as bad or worse, but I don't know of them first hand). For example, Kaspersky's Anti-Virus will refuse to install if you have the ZoneAlarm Pro firewall installed (and perhaps other competitors' products), which is widely considered the most secure on the commercial market.
Kaspersky also refuses to install if you have one of several competitive anti-malware tools installed. The goal is quite clearly to try to coerce you to buy Kasperky's other security products instead. I find that outrageous!
The truth is, however, that if you install Kaspersky AV
first (you have to uninstall those other products before the KAV install will work), and
then re-install them afterward, they usually work just fine together! You may have to disable specific options, such as uninstalling ZoneAlarm's bug-ridden Browser Defender / Toolbar from your browser's add-ons or make sure you don't have two simultaneous email anti-virus monitors going, but otherwise there's no actual conflict or reduction in security like they claim there would be.
That's another important fact: Most vendors and sites loudly insist that you must never, ever have more than one anti-malware tool installed, but I've found that that's almost always a flat-out falsehood. In this regard, Avira -- especially their user fora -- is one of the very worst offenders around! If you post a problem with Avira Anti-Vir, commercial or free, they make you post a "Hijack This" log and if the other forum users or moderators see that you also have a third-party firewall or anti-spyware tool installed, they'll almost
always blame that other piece of software for
everything. They'll order you to remove it all before they'll even bother with you, and even then they'll very probably just insult or ignore you for even
suggesting that there just might be something wrong with their apparently "sacred" software.
As such, I was quite shocked to see some positive comments about Avira's user fora above in this thread because every single time I've asked for help there I've been lied to or otherwise treated terribly. Also, Avira's moderators are considerably more power-mad than most mods.
That being said, I guess it's a bit ironic that I currently use Kaspersky's paid anti-virus on one XP Pro box and Avira's paid anti-virus on the other four XP or Win7 boxes because they've both been highly rated in independent AV tests. And I use the paid versions because you really do get better tools that way: The XP machines that I've used freeware AV tools on have been infected countless times (though of course they never reported that, so I had to learn of these infections through other means), while the machines I've used highly rated
paid AV tools on have been infected far less often.
In the past, I used G-Data's Anti-virus tool which was then extremely highly rated because it used multiple anti-virus engines and therefore had extraordinarily strong detection ratings. Unfortunately, it was quite a resource hog for precisely the same reason: it used multiple anti-virus engines. That's why I switched: Avoiding infections is not the
only thing you want your computer to do!
I find Kaspersky's AV to be distinctly superior to Avira's in terms of detecting and removing viruses, yet I nevertheless use Avira's AV on every machine that I use to download very much because, of the two, only Avira provides an on-demand virus scanner that you can invoke using the extremely useful and popular
Download Statusbar browser add-on. That way, you can automatically scan every file immediately after you download it.
Kaspersky didn't even understand the question when I asked their tech support team about a dozen times if their AV tool provided a similar capability! Finally, a higher-ranking person admitted that their product simply does NOT support that, but he promised it would be added to their 2011 AV product. They're selling their 2011 AV now, yet when I emailed their tech support department recently several times to see if they actually
did add that capability, they simply sent back form letters with nothing but unrelated sales claims and still has never even
tried to answer my actual question.
YMMV, of course....