I used to have trendmicro I used it for free when I worked with a previous company, when I left I had to uninstall it and switched to using Zone Alarm.
I tell you on some days I really want to pull my hair out when my PC really works like a snail and sometimes really so unresponsive for up to 5minutes or more, I'm tired of using ZA and ready for another change even if I have to pay for it.
I have a Celeron 2.4GHZ with 768MB ram, not the newest but with trendmicro it was still very responsive. Before trend I used symantec that in my opnion is another big resource guzzling software too.
So what else is good Anti-virus with built-in Firewall that won't gobble up all my system resources.