It may be a bad DAT push.
Here is a quote from Ex Brit (section SecurityCenter 9), probably worth trying:
'This would apply in 2 instances: 1. When DATs fail and 2. When people get the errors and red X's warning them 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 errors should be all be cured.
(I now have 5567). '
I had an issue removing one particular item in the folder, but forcibly removing that item caused major problems (fortunately I could restore an image). I actually could terminate mcshield.exe in the task manager. That doesn't seem very safe ! Malware could do something similar, right ?
And it's even mentioned in one of those two links. In my case, terminating Mcshield.exe broke McAfee, it claimed McAfee needed to be reinstalled !
Anyway, Ex Brit's solution worked.
Seriously, those FAQs/links should be updated, and McAfee shouldn't send out bad DATs so often. I think I've recieved a lot more than 10 bad DATs (downloading, but not installing) over the past 12 months.