Vista restarts when commanded to shut down

These events have occured since I installed the following updates:

KB947562
KB915597 (three definitions)

I select the shut down icon from the start menu. Vista goes through the
normal shut down sequence which includes ceasing the signal to the monitor
and an external USB hard drive. However, the power lamp on the computer does
not go out. Within a few minutes ( I have not timed this) the computer
reboots. At this point the computer pauses at the power-on password request
and I can either press the power button to force a shut down or elect to
continue with the restart. When I choose the latter and the restart
finishes, Vista posts a notice that it had shut down unexpectedly and cites
Vista as the cause.

The OS recommends at this point that I check for updates, but I have no
updates pending. The "Problem Reports and Solutions" function returns the
notice "No solutions found."

As this is only a minor inconvenience I have not resorted to a system
restore or to uninstalling the updates (I am by no means certain that this is
related to the recent updates). I am hopeful that either someone has a
solution for this problem or insight into the cause. Thank you.
--
BrockTungsten
Man of industrial metal.