Avast worked great for me, but since we had an hacker, we needed a kit. So i got PC-Cillin ,for 40$ can. and it works great!!!! its awsome, and very effecient! But 40$ is money, and if you want free, go with Avast or AVG (recomemnt Avast)
NOD32 uses less resourses (memory and stuff) and detects viruses befor u download them!! also it automaticly downloads the latest virus signature database
I have gone through many AV software packages, but my most favorite is Avast!. I would however recommend you to try the others first so that you chose the one that suits you best.