According to AV Comparatives' tests [1] Avira Free Antivirus performs the best out of all free (gratis) antiviruses for Windows. I use it myself on Windows XP V

According to AV Comparatives' tests [1] Avira Free Antivirus performs the best out of all free (gratis) antiviruses for Windows. I use it myself on Windows XP VMs as well as hardware Windows 7/8 installations and aside from the annoying daily pop-up I found the experience quite satisfactory. The downside is that with real-time protection enabled it slowed down older Pentium 4M and AMD Sempron machines I've tried it on a bit too much compared to Avast. [1] Specifically, http://www.av-comparatives.org/comparativesreviews/dynamic-tests http://www.av-comparatives.org/comparativesreviews/dynamic-t... and http://www.av-comparatives.org/comparativesreviews/detection-test http://www.av-comparatives.org/comparativesreviews/detection.... Avira scored second (out of 15 free and paid AVs) on the file detection rate test, which for my purposes is the most important metric. I'm disappointed that those tests do not feature free and open-source antiviruses like ClamAV, though.