My current favourite is Microsoft Security Essentials. Apart from it being free of charge, it follows that as Microsoft's software is attacked first, they get to identify the threat and update their AV software before letting the others in on the secret.
Both Avast and G Data are excellent standalone programs. G Data has two engines -- Avast and BitDefender -- designed by G Data to work well together. G Data was also the first company to develop an antivirus program. The new Avast also has a compatibility mode for the purpose of allowing it to work with another Antivirus program without any conflicts. I have only tried Avast in the Compatibility Mode with Microsoft Security Essentials and Trend Micro Titanium Internet Security, and they work very well together. Trend Micro Titanium I/S has a firewall booster that can be activated to support and strengthen the Windows Firewall. Other good standalone programs are AVG, Avira Antivir, Panda Cloud, Norton, and BitDefender. The first three of these last ones come in free versions. I installed Panda Cloud on my wife's Chinese language version laptop because it requires no maintenance and updates automatically.