Lightweight Antivirus?

Hello I have a PC which is just 1/2 year old and I am using BitDefender Total Security 2014 as an Antivirus .. but I sometimes use my laptop which is I think 6 - 8 years old and it is very slow and sometimes it hangs. I am using AVG Antivirus 2013 for that laptop and I think that it is the cause of the additional burden for my laptop. So what are antivirus programs that are lightweight and very effective? Because I don't think that MSE is effective because I was using MSE and it was not getting any virus but when I switched to AVG, it got some viruses. ( Im not after free antivirus only ) I need a lightweight but very effective antivirus

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