MSE is absolutely terrible. Working for an IT company, most of the systems we see riddled with viruses are running it. Remove, install BitDefender 2014, scan and clean.
You can get BitDefender for £7 online. I don't see why anyone wouldn't spend such a small amount of cash to keep their system safe rather than a free AV
http://www.av-test.org/en/tests/home-user/windows-7/janfeb-2014/
Very low system resource usage as well, even on older XP machines
Norton is OK, its came on leaps and bounds last few years, but one constant has remained, and that it makes slightly older PCs run like absolute crap. I would choose BitDefender, Bullguard or Kaspersky over it any day.