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#21
Immunet free would be my guess.
Microsoft security essentials and Avira are the lightest two anti virus i've used. Hell Avira is lighter on the cpu then Microsoft security essentials due to my past experience testing it on my bros Pentium 4 pc at 2.6ghz and 1gb of ram with win xp pro and runs better then MSE did since MSE was spiking his cpu.