I use Avira Anti-vir free.
About three or four times per year huge updates beat down the free versions servers,and updates other than
manual,fail with great regularity, at least in Texas.
At that stage I get enraged, join the course of Bitching and Moaning on the Avira Forums,and pull it off my system,to
try AVAST!,AVG,30 day trials of Nod-32,Rising Av,Twister Av,Comodo AV,et al.
I always thank I have at last found the light,and I always
end up back with Avira.
I know the interface like the back of my hand,Its light and stable,and I trust the protection. (with Malwarebytes anti-malware,SuperAntiSpyware,and A-Squared Free,on deand scanners,along with Threatfire and Sandboxie.)
Now, if I was not so cheap, I would use Nod-32.