I have used nothing but Microsoft security essentials (100% free download fro Microsoft)on my windows 7 PC right from launch and have never had any issues. I check every now and then with malware bytes and it has found nothing there that was missed.
I used to use Kaspersky paid as I was tired of AVG giving false positives and missing stuff. However MSE does just as good a job in my experience and is free.
I agree that the user is most often the weak point, but can only speak as I find as far as MSE goes and for me (I don't do stupid dodgy searches) its been rock solid.