Avast Antivirus historically has one of the highest rates of false positives of all antivirus products tested by AV-Comparatives. Avast Antivirus also has a history of connecting with Google servers to send analytics. The latest version of Avast Antivirus defaults to inserting a promotional advertisement into your outgoing emails. That's the final straw.
- Low CPU use during real-time scanning
- Low memory use
- High rate of detection
- Low rate of false positives
- No inserting ads into outgoing emails
- No intrusive advertising popups or nags on the desktop or system tray
- On-demand and real-time scanning
- No (or minimal) extra bulk
- Can be uninstalled cleanly
- No connecting to Google Analytics or other similar operations
I am not concerned about CPU use during on-demand scanning.
This question is open to answers for any version(s) of Windows. Compatibility with every version (such as XP) is not required.
Based on these simple requirements, the following do not qualify (based on current AV-Comparatives testing): Avast, AVG, Trend Micro, Baidu, F-Secure, ThreatTrack Vipre, Fortinet, Panda, Quick Heal, Lavasoft, and BullGuard.
Currently testing Avira Antivirus on that system. It has excellent ratings and could possibly meet the requirements. Unfortunately, it seems to have a serious issue. See this SuperUser QA for details.
Avira Antivirus appears to be "scareware": almost every time you boot, it displays a message trying to scare you into buying their premiere product. It may display these banners at other times as well.