I bought Kaspersky (v.7.0), which worked out to free after a rebate, and it's mostly excellent. Its performance vis-a-vis intrusion is terrific. I'm still learning how to most easily allow what I want to work, to work without my prompting it. (75% of that was easy to set immediately, and another 20% within a week -- it's those last few things that it does that I'm working on, a few months in.)