I get asked from time to time about free antivirus software recommendations..

Almost all the "free" antivirus software out there is really a trial or time limited version trying to, in the end, sell you something. There is one truly free and very good free product out there:

http://www.clamwin.com/

Do you recommend it? Rarely, let me explain..

ClamWin is a quite good virus SCANNER, however that's only the main feature of the commercial competitors - ClamWin can SCAN for viruses on demand (when you ask it to), or on a schedule.  This only partially protects you.  For example:

Let's say a friend gives you a USB key, you plug it into your computer, it auto launches a virus installer silently (you don't know it).  Would ClamWin protect you? NO! However if you plugged in the key, had auto run 100% disabled, and manually said scan the usb key it would detect most any virus.  However in this case it would be too late, you'd have the virus and then be relying on it hopefully removing it.

What do you recommend?

ESET Nod32 in almost all cases. There are exceptions, but unless you have a complex need / large network to manage just use this one.

What's different about ESET Nod32?

This is a real time scanner, like all the major commercial anti virus vendors. What this means is given the same situation above, when the infected USB key is plugged in and the auto run program attempts to start Nod32 would first scan the program - It would detect it's a virus, before it ran, and alert you.  This would stop the virus from being installed on your computer.

There's a HUGE difference between real time scanning and on demand scanning, both in the level of protection AND the cost (free versus commercial software).  ESET Nod32 is quite affordable though, pricing varies by number of seats purchased. Pricing is under $3/Month ($58.99 for a 2 year subscription).