AVG Free is the BEST
Reasons,
1. 100% free PC protection and you can add Internet Security for very less cost
2. An option to enable immediate Automatic USB Devices Scan at plugging
When you plugged in a USB device to your PC, AVG will start to scan it automatically
3. Resident shield
Auto scan, files and folders when you open, (if you double click a virus, no harm to your PC, AVG will block it in a second with a warning and will ask from you to delete)
4. Auto update with less data usage
I use AVG Free. I have monitored its weekly auto updates and it use approximately 500KB per week for update
5. No affect to system speed
I haven’t feel any affect from AVG Free to my PC speed (I’m using Core 2due, 2GB ram, Windows 7 ultimate)
6. Version update 100% free
When AVG release a new version, your older version will ask from you to auto update for free
If you need to add Internet Security (I think it is essential to a personal computer), You can buy it with 20% discount following the below link,