In Windows 8.1, 10 and above Windows Defender contains an antivirus which means it should not be on when a 3rd party antivirus such as McAfee is installed. Using 2 Anti-Virus applications together can lead to major issues.
With McAfee installed Windows Defender should be automatically turned off.
It may be the case that your software has become corrupt.
Try completely uninstalling all McAfee products via Control Panel > Programs and Features.
Reboot if asked to.
Run the MCPR cleanup tool: http://us.mcafee.com/apps/supporttools/mcpr/mcpr.asp
Go to your online account, download and install from there.
Hint: Save time by downloading the installer first, save it to your desktop. Right-click it and go to Properties. Unblock it, apply etc.
Then doing the above except now you can simply install from the downloaded installer
Please kindly inform us if this resolves your issues.
All the Best,