Download & Install the following 3 freewares. It forms a free complete security suite for your system!
1. Avira Anvivir - http://www.free-av.com/en/download/index.html .
Avira is best & light solution for internet security, real time updates, excellent overall system protection.
I would add Immunet Protect to this list. It is free and has real time detection and protection.
It's pretty cool AV software-- once you download it and become a member, you are a part of the Immunet Community and gain "Collective Immunity", meaning if you register a virus, everyone else within the community is automatically protected from that virus.