"Windows XP and Vista have started installing updates at shutdown, in certain cases, without displaying a warning or requesting permission, according to reports by several readers.

The forced-install behavior has been witnessed at least three times by Windows Secrets editors, but Microsoft says its procedure for Automatic Updates hasn't changed in the last 10 months.

The behavior seems to occur only if a Windows user has Automatic Updates configured to "download updates but don't install them" or "notify me but don't install them." If updates are scheduled to occur automatically, with no notice to users, the silent installation of updates would be expected."

Full story: http://windowssecrets.com/2009/06/25/01-Windows-may-install-updates-without-asking/#story1

Comment: Just another reason why I prefer to manually update at MSU.