I switched from Norton (5 licenses) to MSE for a couple of reasons.
First, Norton was slowing down the system and consuming huge amounts of memory. Not so much a problem on Win7 but on my older XP systems with limited CPU and RAM it's a killer.
Second, Norton charged my credit card for a subscription update without me initiating the transaction. They called it "enhanced customer service", I called it "you just lost a customer". Used to be when a subscription ran out they nagged repeatedly and pushed you to their website where you had to acknowledge that you wanted to buy another year's sub. Now the default is "we charge you automatically". You can opt out - if you know about it.
These days I use Ubuntu about 90% of the time but for the 10% that's spent on Windows 7 MSE seems to be working (no hits yet so hard to verify that) and the resource use is much less intrusive (so far). I still have Norton on one system (the one I paid for automatically) but when that runs out - I'd done with Norton.
I've used some other AVs, best left un-named, but their sig update process was so unstable I had to ditch them.