Hey
I have had CIS (comodo internet security) Avast combo (comodo AV's realtime protection/scanner was off of course) and comodo has never used alot of cpu (cmdagent.exe) during the time I used this combo. Maybe i was just lucky
Zonealarm may be good but comodo is a better option; comodo has HIPS-technology which provides you with better information and you get cloud scanner as an extra.
I recommend all to check the download program(s)/application(s) before are getting installed on the machine, at least I do.
Regards,
Tenko