I've been using AVG Free as my anti-virus software for well over a year now. Having had nothing but problems with Norton products, and not liking the myriad processes and services invoked by McAfee, the more configurable AVG has proven a natural fit for my behave-wisely-and only-scan-when-something-seems-amiss philosophy. And, as you might've guessed from the name, it doesn't cost a dime.
My first impression of version 8 was that the company had finally got its GUI act together. Gone were the separate control, test, and vault applets and multiple settings dialogs. All the functions are now under one roof and settings melded into an expandable tree/tab control that makes adjusting the program far more intuitive and far less time consuming. Also, the new look introduced by version 8 is far more attractive than the old one, which often reminded me of Windows 98.
At first blush, version 8 also seemed quite a bit more effective than version 7, though updates to the version 7 virus database helped with that older utility quite a bit when I tested both side by side, as well as versus online scanners such as Trend Micro's House Call and Bit Defender. Unfortunately, as with most AV programs, AVG's false positive rate seems to have gone up as well. That said, AVG8 seemed about on par with the rest of the pack in that regard.
AVG 8 Free doesn't offer the rootkit or full spyware protection that the pay version has, and support is limited to the documentation found on the site and in user forums. However, the free version does offer a new toolbar for your browser to help fight off attacks from that quarter.
For those who share my minimalist philosophy, AVG is still configurable to the point where you can disable it and run it only when you think you've been invaded or have downloaded something suspect. And even when it is running in full auto mode, there are only two services and a system tray applet draining minimal CPU cycles. Alongside the equally low-profile NOD32 (which isn't free) and Antivira, AVG is my favorite AV software
Tip: When things are really a mess, boot to Windows Safe Mode (press F8 when booting) and run AVG 8. As do other AV programs, it will then scan using the command line interface with no nasty services running that might interfere.