Avast is good.. I was a big fan of nod32. But it sucks when dealing with regsvr virus and i stopped using it. I dont know whats the condition now.
Avira detects spyware. I was reading about Avira in a review, it was the fastest and used the least resources. I use Spybot Search & Destroy. I strongly recommend it. I WAS using SystemSuite 7 from Avanquest, but after only three years the stopped providing updates for the antivirus !
I still go for NOD32 when it comes to the best anti virus today but if you don't have the budget on buying such an anti virus program like NOD32, the best anti virus that is free is no other than AVG.
The best is Avast antivirus and it got updates easliy unlike the norton takes a long time. And it is free home version available.
In my viewpoint all antiviruses are not fully capable of stopping the virus activity but Symantec Antivirus Corporate Edition for windows xp is amongst the best to deal with viruses and spywares