I'm sure everyone will have their own personal preference when it comes to AV solutions. I currently use a paid option, Webroot purely because it only cost me a few quid in a recent promotion and uses such a tiny footprint.
For free MSE seems popular - http://windows.microsoft.com/en-GB/windows/security-essentials-download
To compliment that I'd recommend Malwarebytes and/or Emsisoft - http://www.malwarebytes.org/products/malwarebytes_free/ or http://www.emsisoft.com/en/software/antimalware/