The most popular free Windows antivirus programs are : AVG , Avira and Avast ;
In terms of price-performance , they are the best !
Many people think that if something is free then is must to be worthless;
It's not the case with anti-viruses, because the business model is free for home use
and paid for enterprise. The anti-virus vendors knows that the piracy is high on the
consumer side, and if you can't make money on this side, at least make a little bit of marketing ...
. . .
There are lots of paid antivirus programs out there such as Norton AntiVirus; Kaspersky Anti-Virus, and McAfee AntiVirus Plus, but there are lots of good, free antivirus programs out there as well.
Full security suites : BitDefender Total Security 2011; Kaspersky Internet Security 2011; ZoneAlarm Extreme Technology.
Beside the security programs above there are other top-rated Internet security suites.
Also reliable tests are made by AV-Comparatives.org and ICSA Labs.