I've used @Guard, ZoneAlarm, Norton/Symantec, Tiny, Sygate and Outpost; I found they all worked fine - excepting ZA v5.0, but they fixed that.
@Guard and Sygate are now Norton/Symantec and Tiny don't offer a free version now.
I use Outpost v3.5 now and it seems to be doing a great job; it detects a lost of ad/spyware and it's blocking facilties are terrific.
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