I run Windows 8.1 and use the built-in Windows Defender (same as MSE). I have heard that many would attest that it's been declining in quality although I don't do anything to attract malware so it's good enough.
See this on computer security if you want to learn more (EMET will hinder trainers so take caution if you do use it). An anti-virus is actually an incredibly small defense mechanism compared to what you should be doing to secure your PC
Link
[Edited by Neo7, 4/8/2014 10:34:14 PM]
I can vouche for the sight Neo has linked.