I would suggest uninstalling McAfee first. then you can modify the registry to turn Windows Defender back on.
Open Registry by typing in regedit user search or run.
I would strongly recommend backing up the registry before you make any changes by clicking File > Export.
Navigate to HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows Defender
Locate entry on the right side DisableAntiSpyware
~if entry doesn't exist right mouth button click. Select New> DWORD. Name it DisableAntiSpyware.
Double click the DisableAntiSpyware entry, under Value Data change the value to 0.