This could indicate a problem with your powermanager.
Try reinstalling your powermanager or...
There is a registry string
called "PowerdownAfterShutdown" wich you can find at
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows
NT\CurrentVersion\Winlogon
Try change this to a different value. If it is 1, change
to 0 and vice versa.
Maybe your computer doesn't poweroff automaticly after
this but it won't restart.