I'm running
Bitdefender Antivirus Free Edition on both my WIN 7/8 Netbook, and WIN 8 Ultrabook SSD.
Bitdefender received the highest rating along with Kaspersky in WIN 8.1, while being lighter on resources than Windows Defender(8/8.1), formerly known as Microsoft Security Essentials. Also, perfect score for protection in WIN 7. It's the most un-intrusive Free AV software I've seen yet.
Microsoft Security Essentials/Windows Defender(8/8.1) scored at the bottom of all AV software tested for protection in
WIN XP /
WIN 7 /
WIN 8.1.