I recommend that you look at the most recent comparative tests. See this, from April 2014:
AV Comparison Summary.
Well I looked at the summary as suggested by SD and decided I would try BitDefender free version.
All I can say is 'I wish I hadn't!' The BitDefender software is very reluctant to load and if it does, it's only after it has installed a number of crapware and scareware programs. If you decline the offer of these items it seems you won't get to the BitDefender installation at all. I've just spent most of the afternoon getting rid of all this rubbish and reinstalling Avast which actually seemed to be doing a reasonable job.