Microsoft offers free antivirus protection for Genuine Windows users.

Microsoft Security Essentials is a free antivirus program for those who have genuine copies of Windows. As I'm not using (at all) any OS from MS I wasn't able to test it, but I can tell you some details and what is being told about it. So, what's good is that it is free (of course, if your copy of Windows is licensed) and it is made by Microsoft, who knows exactly what's inside the operating system so they can fix any vulnerability faster than a regular security company.

They say that it protects your computer against viruses, spyware and other kinds of malicious software, all in real-time. What I think that is bad, simply because it is made by Microsoft, is that it may fail regularly in different situations (fake alarms, issues when running, weak detection and stuff). But this is not something proven and is based on my past experience with Microsoft's products.

Those who tested said that it detected and resolved several threats better than NOD32, and even used less resources, but I doubt that this is something relevant (as I haven't seen any "serious" tests). There are comments on blogs where this product was reviewed in which the people told that it is a very bad protection program, because it wasn't able to detect important trojans/viruses so my doubts come also from here.

You can download and try Microsoft Security Essentials from http://www.microsoft.com/Security_essentials/ .

iAntiVirus - free malware protection for your Mac.

As we all know, Mac is a really safe OS but the truth is that no platform is immune to any kind of malware. Even the Mac OS X. However, the risk on infection on Mac is really low, since most of the attacks are directed against Windows. So there isn't a real necessity to use an antivirus software if you're on Mac, but if you have "terrifying" experiences from Windows, you may use one.

iAntiVirus is a free antivirus solution for Mac, offering most of what a regular user needs. As the official site says, this application offers powerful malware protection (viruses, worms and trojans) - detection and removal-, spyware, adware and dialer protection, real-time protection via "IntelliGuard Real-Time Protection", free support by community and free "Smart Updates".

It offers just a basic interface, and uses very little resources as I even don't feel it working in the background. These are what the free version offers. If you need it for business and commercial purposes, 24/7 dedicated and phone support, paid version is also available.

Consult the official site for more details: http://www.iantivirus.com/.