I previously installed Avast as sole AV on over a hundred machines without issue or infection.
Then suddenly Avast couldn't detect a fake virus scan infection on my Dad's computer, even running it's Boot Scan with no services running where virus can hide. Ditto another friend's Avast computer a few days later.
MSE found and removed these up-to-the-minute infections from the desktop.
I am now installing MSE instead of Avast and find it to be even less intrusive than Avast - which just slimmed down even more with no bothersome registration, probably to compete. MSE is nearly invisible once installed.
My experience with paid AV's is they are bloatware. Norton is like a hijacker infection which requires special tools to remove similar to the worst Trojan. Having Norton and Office trial ($260 bill arrives 60 days later after you're hooked into it) preinstalled is an atrocity upon Windows 7 requiring Revo Advanced uninstall or (better yet) a clean reinstall if you have unfixable system files after running sfc /scannow.