Try the following:
This would apply in 2 instances: 1. When DATs fail and 2. When you get the errors and red X's in the taskbar or Security Center warning that VirusScan needs reinstalling.
Boot to Safe Mode by tapping F8 while booting up.
Go to Program Files\McAfee\VirusScan\DAT and delete the contents of the folder, not the folder itself.
Reboot. Ignore warnings that Security Center may throw up.
If updater doesn't start automatically, right-click the taskbar icon and select Updates.
Let it do its thing and when finished the problems should be all be fixed.