In the reviews Windows Defender works about 95% as well as AVG free doesn't have big impact on performance, but it harasses you a lot less with nonsense, so it's up to you. I prefer to just be careful and not be bothered so I just use Windows Defender.
http://www.pcadvisor.co.uk/buying-advice/security/3456902/best-free-antivirus-for-pc-or-laptop/