MSE is my personnel choice. I had installed it on my Sister's laptop with auto updates turn on. Its running peacefully since past 1year without any annoying popups. what's better she didn't even notice it, until it blocked malware running. Tension free solution for free. Kudos to Microsoft. If my Linux machine needed an AV software(Fortunately it does not) I would choose MSE.
And those to are suggesting Avira or Avast - Let me point of flaws of each.
Avast Free - Its malware detection engine is poor.It gives false positives & sometimes altogether fails to detect malware. Sometimes, It passes a infected file as clean which is nasty!!
Avira - Good AV with Strong detection engine but Its an adware.Can't tell you how much I hate those nagging popup window which blocks sreen everytime Avira updates itself. I fcking hate it as It stops work that I am doing and distracts me. It comes on the screen automatically out of nowhere. worst, there is no way to disable that nagging popup.
If you need a Basic Protection, MSE is way to go. If you need another option. Check AVG free, its decent too