Over on the well known Raymond.cc blog, Raymond has posted an extremely comprehensive, detailed and useful list of 26 different anti virus programs - all available for free. Some of these are well known, others less so - and still a few you may have never even heard of.
In each case, Raymond actually installed, setup, and ran each program - going all the way through the installation and downloading of the latest definition updates. He mentions when certain programs attempt to change the user's system during installation - such as add toolbars, change the home page, and even the DNS settings, as in the case of Comodo.
"Billions of dollars are spent every year on antivirus software alone by enterprise because they know that computer virus can cause a lot of down time and making the company lose money...
As for home users, most antivirus companies also offers a cheaper version that works on Windows operating system and requires an annual subscription. However, there is also a wealth of anti-virus software offered for free with the catch of disabling some of the extra defense system such as firewall, behavior blocking and also only allowing you to use it for personal home use only. After extensive research, we have come up with an up-to-date comprehensive list of free anti-virus programs for your convenience."
I think many people here will find this article to be very useful - and possibly a bit controversial, in that - when it comes to anti virus programs - it seems that opinions seem to differ greatly.