I will go with AVG. It is the best suited antivirus I have used.
I THINK Guardian AntiVirus 2012 (A quick heal products ) is best xp and other systems........
I have been using Kaspersky for long days. Quite satisfied with its performance.
I think Avast is good.
according to me best antivirus is avast...
I use AVAST antivirus
antivirus are not more important these days
I am using AVAST right now and so far I do not have problems with it. My computer is very well protected and also it is quick in detecting some unusualities