AVAST is good ANT and being used by mostly users, I was also using it but from few days I am using (360 Total Security) as recommended by some Digit friends .The merit are as follow:-
It has 5 Scanning Engine's as follow:-
1. 360 Cloud scan Engine
2. System Repair Engine
3. QVMII AI Engine
4. Bit-defender Engine
5. AVIRA Antvir Engine
So above three Engines are in Built ,But last two we have to update if we required it depend upon us. Its also very light on resources, But may be cost 200MB updating last two...