AVG Anti-Virus Free Edition 9.0 is basically the best anti-virus you can get for free, even compared to Microsoft Security Essentials, which is also free.  C-Net and PC World have reviewed it and have given AVG Free very high marks.  This AV is very low on RAM munch, only using about 40-50 MB of RAM, compared to the usual 80-100 MB of RAM compared to Norton and McAfee.  I have installed this on my wife's, mother-in-law's, grandmother-in-law's, and my own PCs.  I have tried multiple AVs out there, including Avast, MS Security Essentials, Comodo- and find this is the best of the free AVs.

AVG Free site