To clarify something about antivirus software.
A best antivirus does not only keep your pc clean from online or offline threats, but it keeps your system at stable condition, as well. This means that it should feel as a lightweight program.
I do not know which commercial anitivirus scores on the last part. Kaspesky or BitDefender never felt lightweight to me(Though, I consider Kaspersky as a top AV & Anti-Malware software).
The only true light anitivirus is Microsoft Security Essentials from my experience.
It is not as good as a commercial antivirus, but its decent at least. You may add a secondary software for anti-malware purposes such as MalwareBytes or HitMan Pro.
You protection is enhanced with a combo (Main Antivirus + Secondary Anti-Malware Software).
Both of them are cheap and lightweight. They will cooperate fine with other free or shareware antivirus (MSE, Avast, Avira, etc) as long as you tweak the settings in Exclude section of your main AV.