Personally I've seen and have used free Anti-virus software and I've always returned back to Norton anti-virus
it just seems to be written with a little more strength and power to it.
If you go to your local computer store and ask for a OEM version of Norton Anti-virus 2008 its only 20 bucks and gives a valid year of updates
20 bucks for a year of solid security seems to be worth it.
I found that it doesn't drag down your system if your computer is recent, unlike it used to years ago.
I actually use NAV 2008 and Spy-ware doctor at the same time with no problems, and I feel well protected.
Just my 2 cents.