Hi Sir,
My suggestion is you keep using avast! antivirus with your machine, because according to your tech specification that you using quite low technology same as like my home machines.
Norton always claimed themselves not need a huge resources to run it, but again you should specific to analyzed it again what is machine spec when norton trial it?
Because each AV vendor always claim that they can running heavylight on machine, but in real you will see what will happened in the field..
cheers,