Antivir is the best of the free antivirus software applications, by free i mean free for personal use.
If this is for business use, your only choice is Clam, for it is under a GPL and is an opensource application.
Antivir, according to every detection rate test i have seen recently, has a higher detection rate than the other free ones (AVG, aVast!, and ClamWin). It also uses very little ram and system resources. It does not have an unattended update but you can configure it to update itself with about 3 clicks from the tray icon.
please see this site for detection rate tests:
http://www.av-comparatives.org/
Another antivirus detection rate test of 91202 virus samples:
http://www.virus.gr/english/fullxml/default.asp?id=69&mnu=69
AntiVir version 6.30.00.17 - 84.50%
Avast version 4.6.623 - 79.65%
AVG version 7.0.308 - 54.07%
ClamWin version 0.83 - 48.44%