Are you looking for a free Anti-Virus for your PC? Look no further, there are two simple solutions.
If you are running Windows 2000 or XP, there is Grisoft's Free Antivirus... It may be lacking some of the features and It may be annoying a bit... but it works, and it gets updated regularly. Updated: There is a new version available out there, so if you already downloaded one, be sure to download the updated one now.
If you are running Windows XP, then you have an option of using Google Pack. Follow the link on this page and you could download Norton Antivirus 2005 which is part of Google Pack for free.