Antivirus (Free and non-free) recommendations

Home users:
For most home users, you're best off just using a free antivirus.  Yes, the pay versions include fancy firewalls.  But generally, you don't need them.  If you have a hardware firewall and enable the software firewall in your OS, you'll be fine.  (Windows XP and newer all have software firewalls.)  

And most retail antivirus applications are bloated, slow, and just plain awful.  Users would have me look at their home systems, complaining how slow the systems were.  I'd remove Norton Antivirus, and poof! -- performance increased dramatically.

(Yes, real system administrators don't let friends use Norton Antivirus.  I have a friend whose husband works for Symantec, but I gave her the same advice.)

My favorite antivirus for home use is Avast! Home Edition Antivirus.  It's fast and low-resource.  It also checks incoming mail, websites, IM clients, and more.  You can't schedule scans and that's about it versus a full pay client.  I use it on most of my home systems.

http://www.avast.com/index.html

The only caveats with Avast!:

• The sounds it plays (especially the virus database update) can be really annoying.  Especially if they blast out of your speakers at 4am in the morning.  But they can (and should!) be disabled.  Info here: http://answers.yahoo.com/question/index?qid=20080107133619AAQYb5I
• Avast requires registration with them to work beyond 60 days.  Avast then emails a key good for 14 months of free updates.  LOTS of users ignore the warnings that their subscription has expired and never renew.  Remember this if setting this up for Aunt Petunia.  (Stick a reminder in your Outlook or Google Calendar on this, as Aunt Petunia will fail to renew...)

Microsoft's Security Essentials seems to run quite well, too.  No registration is required, no renewals are required, and it also is very fast and low resource.  But it doesn't check incoming mail, websites, etc.  It's therefore not quite as comprehensive a product as Avast!.  

http://www.microsoft.com/Security_Essentials/ 

Home Businesses and Businesses:

The list of antiviruses that are totally free for business is pretty slim. Most free antivirus programs are only free for personal usage.

Security Essentials isn't free for business, just home businesses.  But most free AV isn't supposed to be used by home businesses either, so Microsoft is actually a bit more open than most AV companies. Security Essentials' license information here:

"You may install and use any number of copies of the software [MS Security Essentials] on your devices in your household for use by people who reside there or for use in your home-based small business."
http://www.microsoft.com/security_Essentials/eula.aspx#mainNav

ClamWin (which can't do realtime scans), Comodo, and PC Tools are all OK for any use (commercial or personal).

• Comodo is probably the best of the bunch, but it's very, very complex. It comes with Comodo's software firewall -- which is very powerful but arcane. 
• PC Tools' antivirus doesn't automatically update, making it only quasi-useful.  
• ClamWin uses the open-source ClamAV engine.  It's pretty good, but not great, especially when it comes to virus removal vs. simply detecting.  I've had very good results, however, using ClamWin to check incoming emails for mail servers.  It works great for that -- the ClamAV site admits that's it's primary function:
  
  "Clam AntiVirus is an open source (GPL) anti-virus toolkit for UNIX, designed especially for e-mail scanning on mail gateways"  http://www.clamav.net/about

http://www.clamwin.com/
http://antivirus.comodo.com/
http://www.pctools.com/free-antivirus/

Spyware Terminator is completely free for any use and can use the ClamAV engine as a realtime antivirus. But I haven't tried it yet. This may be the best current option.

http://www.spywareterminator.com/
http://www.techmalaya.com/2008/05/10/clamav-real-time-scan/

Moon Secure AV is a realtime version of ClamAV, but it was very flaky the last time I tried it. Maybe it's better now? (Wikipedia notes that Moon is violating the GPL by not publishing its code. So take that into consideration, too.) If Moon worked well, it would probably be a great choice.  Here's hoping...

http://www.moonsecure.com/

So for real businesses, you'll probably need to fork out the cash for a real pay antivirus.  My favorites:

Eset Nod32 Anvirus Business Edition:
http://www.eset.com/products/nod32_business.php

Sunbelt Vipre Enterprise:
http://www.sunbeltsoftware.com/Business/VIPRE-Enterprise/

There are other advantages here as well.  Real business/enterprise AV programs have a central administration system.  These can:

• centrally download updates, instead of having all your systems separately download updates, thus keeping Internet bandwidth lower
• central reports of infections/detections
• centralized/push installs
• centralized reporting (Did Fred turn his AV off again?  When was the last time his laptop updated?)

What NOT to use:
I've used a LOT of different AV products at this point.  Generally, McAfee and Symantec products (except for maybe Symantec Antivirus Corporate Edition) are terrible.  They're bloated, slow, resource hogs.  Yes, they protect you, but they destroy system performance.

I previously used TrendMicro's enterprise products and was happy to recommend them.  Then quality took a nose dive in 2007 or so.  Updates and scanning went from fairly low resource affairs to making performance take a 50-100% hit.  They fixed some of these issues eventually, but they meanwhile raised their prices as well.  I ended up switching my 300+ node corporate network to Nod32, saving money and a lot of exasperation.

But this is a very dynamic market, and new products arrive constantly, and some products do improve.  Symantec's home products are supposedly much better than previously -- but since Avast! and MSSE are so good, I don't see the point in paying for home antivirus protection.