Avira Free without web protection is the lightest that I have tried, runs excellent on my old XP 128MB of RAM, Intel Celeron single core processor 1.2Mhz. This system would hardly even boot using most other AV's.
If it helps, avast! seems to be working perfectly, and is fairly light on my system. Haven't felt any System slowdown and It's been up and running with all shields fine. Within a day of Emsisoft IS, I noticed a huge system slowdown so well done to avast! there.