I've just started using MSE on the laptop in preference to McAfee, following a catastrophic failure.
Sitting in a guest house room with a dodgy wifi connection in a third world country, McAfee decided it needed to upgrade itself. Part way through the network connection failed. McAfee cleverly decided that this meant that I never wanted to use the internet ever again, and screwed up the machine completely so that I couldn't connect again when the wifi came back.
I got back online (without McAfee) after a couple of days of fiddling around, but I'm not going to run any software that crap again if I can help it.
Look, the whole point of anti-virus stuff is to improve your uptime, yes? If it gives you guaranteed 100% downtime then it hasn't worked, has it, and I'd be better off with something that detected slightly fewer viruses but didn't render my machine useless, no?