Well....Still trying to find the holy grail of the antivirus. NIS 2008 does a good job, But is heavy on non dual core computers (Even 3ghz cpus).
Avast is not good enough. Got 2 infection last week in 2 different computers. Avira's Antivir DID recognise 1 of the infected computer files PRIOR to running it in a test i've conducted.
NIS 2009 is faster than 2008 and works fast even on my super-computer (Sempron 3400+ 1gb).
Had a client with infected system while NIS 2009 gave errors after installation which made me remove it, NIS 2008 did removed the infected file.
Another system was dis-infected by NIS 2009 without any problem.
So from my point of view, Best free non-firewalled solution is Avira's Antivir + Windows defender (+ annoying comodo if you need firewall).
Nis 2009 is best for a non infected system for total protection.

P.S. my friend told me he uses pctools free AV on his clients for the past 6 months, But needed to disable the email plugin, Since it made some problems for 2 of his clients.