Question moved from "Getting Started In The Community" to VirusScan, which seems more relevant.
This is the old "Updates-during-scan" problem, which got fixed and is now broken again. I complained about it in the conference calls but I see no movement to re-fix it, so I suggest you do what I did, namely -
Go into Security Center.
Select 'Automatic Updates' in the top section
Click on Settings
Click on the blue button to turn off automatic updating
Of course, this means you have to remember to get McAfee updates yourself, at least once a day (I do it just before I power down for the night - or, as now, when dawn breaks)
You will be slightly less secure than before, but only slightly if you don't forget to download updates. And you won't be infuriated by having updates suddenly installing while a scan is running.