That's because it has to complete its task in whatever section of the registry it was at and then produce a report and often it takes a long time to do that. In fact at some stages a scan of any type can't be cancelled and that's why sometimes the Cancel button is greyed out.
To make sure your software is OK please run the Virtual Technician: http://mvt.mcafee.com
If any further problems I suggest calling Technical Support, it's free by phone or online chat see the link below.
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