Windows may forced-install updates

 

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" If updates are scheduled to occur automatically" in yours update settings then you will not notice the difference but if you set the windows updates settings as "download updates but don’t install them" or "notify me but don’t install them." then you will notice the forced-install updates or silent installation of updates at shutdown in specific cases may be common so far not known in Windows XP and Vista  systems.

source:Windows Secrets Newsletter  Issue 203