Antivirus

I just upgraded my PC from Windows 7 to the new Windows 10.  Everything looks fine but would like to know if I have to download a antivirus for protection with windows 10 and if so what is a good free one?  I had Microsoft Security Essentials with my windows 7 and loved it but not sure if it will work fine with widows 10.  Does windows 10 already have a built in antivirus and if so where is that?  I've tried looking everywhere and can't find anything.  I have Malwarebytes installed but want to make sure I'm protected with a good antivirus.  Any help would be appreciated.  Thanks.