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.
I use McAfee and it sometimes slows stuff down. But it periodically rescans every file on my system to catch newly discovered malware. Are these products that cause no performance issues simply not rescanning?
NOD32 (pay) - light on resources - very good
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There's also a nice, lightweight, and very efficient anti-malware appl, called Sandboxie, used to be free but not now, which creates an isolation "wall" between a certain application, example your browser and PC's hard drive. Whatever is downloaded from the net, spyware, viruses,... remains in this sandbox, and can be deleted when you close your browser.
It can also be used to isolate other programs as well, which you wish to have them contained, say you want to test a new software. It creates a sort of virtual environment. It's sad that is only for windows.
Sandboxie - (pay) - very light on resources - very good.
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I use Norton 360 v2. In the DOS era I sort of admired the Norton brand but as someone pointed out that is gradually eroding with the Symantec brand. My experience is that N360v2 is not quite resource intensive and I did not notice significant slowdown, but I'm not too much impressed either.
But it did corrupt the jscript.dll on my Vista and causes IE7 and Vista sidebar widgets to malfunction. I needed to do a system file checking manually to fix but that was not a great deal so long that can be fixed.