Avira Is the lightest, There is also Pc tools , which is real good. But Antivir/Avira in my opinion is the best free one..
Nod32 should have a free 30 day trial. Definitely my number one choice when it comes to anti-virus software, just due to the small footprint, and how effective it is at running in the background. I'd recommend at least giving it at try, but the above recommendations are great choices as well. Just steer clear of the big name companies with their bloatware.
I never got on well with AVG or Avast. I use a-squared free now for antivirus and the routers firewall.