I use the free versions of these on Windows 7 http://www.superantispyware.com/ https://www.malwarebytes.org/
I don't know if I've been naive to Microsoft's marketing of 'Defender' but I have never used any third-party anti-virus since I've been using Windows 8 - never had a problem but to be honest I've learnt to detect what is and isn't legit. "You need to install this random .exe file in order to view this webpage." "I oreight."