Fry's B&M has a great deal until on NOD32 Antivirus v4.0 for a 3 User License for $29.99. Regular price is $69.99. Online 1 pc price direct from Eset is $39.99. Here's the ad page from the Dallas Morning News.
With a more serious, non fan-boy tone, I think Avast is plenty light. And at zero dollars, it's great value. I work as a freelance computer technician and I put it on every computer I work on. My clients love the transparency. (The gaming mode, as they call it). Absolutely no popups or other interference... Very professional!
MSE and Avast are free, and have equivalent functions. NOD32 v2.5 and v2.7 used to be light and efficient. I don't think they are light any more.
I had been using Avast in a similar fashion myself lately, but with MSE 2 being out I'm running it through its paces as well.