Moved from Community Interface to VirusScan for better attention.
Windows Defender in Windows 8 should be off when another antivirus is installed.
Is your System Clock accurate both time and timezone-wise and have you tried updating McAfee?
If that doesn't help try running McAfee Virtual technician http://mvt.mcafee.com/ to see if it fixes things.
If not contact Technical Support via the link under Useful Links at the top of this page, it's free.