Avira FREE
Microsoft Security Essentials
MSE hands down. It's awesome.
I suggest you to use Malwarebytes Anti-Malware and Avast
Free: Avast + Malwarebytes
Performance: None
AV is getting passed these days.
I personally use Avast. Some of their features (Sandbox) has saved me many hours of hell.
I usually have my antivirus off than on at most times. It has deleted too many of my built exe's :/. I was one of the very few unlucky people for whom Avast would always reset and clear all white-listed directories. Thankfully its now fixed but old habits die hard.
AVAST is the best. Secureful and simple.
Sandbox is the best freeze computer
Crypt and you can bypass any av with a decent paid crypter
AVG is the best antivirus I've known so far.