I use the free Bitdefender on all my computers. It just sits in the background with no interference and it's top rated by many tests.
http://www.pcmag.com/article2/0,2817,2460688,00.asp
http://www.pcmag.com/article2/0,2817,2421252,00.asp
Here's a list of free anti-virus programs with a quick review and links.
http://askbobrankin.com/update_free_...grams.html?hdr
One thing I suggest is downloading from the original site as others like to add crap you don't need.