Here are my personal opinions for these searching for a good free antivirus:
Avira Free Antivirus (Publisher: Avira) This is the best free antivirus. It has one of the best virus detection database and heuristic engine. It works on Windows 7 or later. The only problem is it sometimes displays advertisement.
Bitdefender Antivirus Free Edition (Publisher: Bitdefender) Easy to use. Good virus detection. But it doesn’t allow user to change advanced settings.
Panda Free Antivirus (Publisher: Panda) Although it received good results for virus detection on some surveys, I personally experienced that it can’t detect some obvious infected files.
Avast Free Antivirus (Publisher: Avast) It is the only choice for Windows Vista users. But its protection is not successful.
AVG (Publisher: Avast) Not so different than Avast.
Kaspersky Free (Publisher: Kaspersky Lab) Seems OK, but not recommended for old and slow computers.