This is a log of free anti-virus that I have used that failed to protect Windows computers so far. Do note that non-free "expensive" ones can also fail. I have witnessed this firsthand in the office where a zero day exploit rendered our network almost useless.
Anyway, this does not necessarily mean that below are lackluster products, it is just that it did fail me one time or another.
List is listed based on chronological order, oldest listed first:
1) AVG Anti-Virus Free Edition - this was my all time favorite but failed me way back in 2010.
2) Microsoft Security Essentials - last week (second week of Nov 2011), one of our home computers was infected by a virus. Got to recover the entire system.
Experimenting with the following for now, see if it does protect a Windows XP PC:
- Avast Free Anti-virus (Program version: 6.0.1289)