The free antivirus program that I used to recommend was
Avira. I still think it's a good program, but I think you might want to check out
Panda's cloud antivirus product
Panda's cloud antivirus software is supposedly very light on system resources. Their "cloud" datacenter of servers keeps track of what viruses are actively in the wild and has their antivirus clients protected for them. It can also attempt to detect new viruses, which they optionally send to their cloud datacenter for analysis. Panda states somewhere on their site that they can have protection rolled out to their entire userbase for a new virus within 6 minutes or so after being reported. It still works even when you're not connected to the Internet, however it works best when connected to the net and working with their cloud datacenter.
Anyhow, I've only just recently installed this panda cloud antivirus on a few friends computers... so far I haven't had any complaints.
Threaded Mode
