AVG Free is good software. It's simple, effective anti-virus protection that's free for home use. The commercial version starts at $20/year/machine. Easy download and install, seems to do the basics well. The UI is a bit clunky but how often do you operate your anti-virus software?
I've been a loyal Symantec Norton Antivirus user for years, paying $25 a year to upgrade. But those upgrades kept adding new features I didn't want and made the product bloated and slow. The trouble started with the worm blocking that made opening network connections 100x slower. NAV 2006 has had several installation and update bugs and more features I have to turn off like the email scanner. The final straw was that its daily update makes my computer slow, pops up a window that steals focus, and apparently can't be rescheduled to a quiet time. Bye.
So now I have 10 or 20 megs more RAM free and my computer boots faster too. Let's hope AVG really does protect well; all the reviews say it's great.