depends; how careless are you? are you or anyone else likely to use the computer prone to visiting sites with the term "free mp3/videos/ porn (...why?)/etc. downloads"?
i've been solely relying on MSE/Windows Defender with a little help from add-ons/extensions in the form of flash blocks, script blocks, and WebOfTrust (WOT), with no problems whatsoever. but that's me. I don't google for "free" stuff or visit suspicious sites.
if you have to, Avira sounds like the top dog lately. Avast from my experience 2 years back, has gone the way of the bloatware. let's not even talk about AVG. for additional protection, get Malwarebytes and do a frequent scan if you're paranoid. pay for it if you can afford it. otherwise the free version is just as good, sans auto detection.