Our current recommendations are either Avira or Bitdefender. These products both have incredibly high detection rates with minimal interference in your computers speed. For those who consider themselves more power users, we recommend Avast.
Those three offer free versions which are far superior to products like Microsoft Security Essentials, and have a much smaller impact on your computers performance than products like AVG.
The most important thing in keeping your PC secure is having software you know how to use. If you have been using the same antivirus for years, keep using it if you know how to use the updates and scans. Nothing will keep your PC clean for you, the more comfortable you are running the software occasionally the better off you will be overall.