I have been using this for years now. AVG has proved to me that they detect viruses the competition does not.
Here is the link to free antivirus protection for home users on a single PC.
http://free.grisoft.com/doc/1
AVG Free Edition is for private, non-commercial, single-home computer use only. Use of AVG Free Edition within any organization or for commercial purposes is strictly prohibited. Your use of AVG Free Edition shall be in accordance with and is subject to the terms and conditions set forth in the AVG Free Edition License Agreement, which accompanies AVG Free Edition.