http://www.grisoft.com/ww.product-avg-anti-virus-free-edition#tba2 Its a free antivirus program AVG from Grisoft. I use it on all of my computers and haven't had any issues. Daily updates, very few false postives, and low resources... Check it out, compare to other options...
Running the OS in Ram as virtual image is also a good method to avoid infection of virus. Once it occurs, just reboot the computer and all viruses will be destroyed because it is only a virtual image.
