Schedule a task with Vista task Scheduler wakes up computer and mo

Hi all.
In Windows XP when i create a new task with the task scheduler and i check
for that task the option to wake up the computer when it's sleeping all is
going fine, that is the computer wakes up at the specified hour performing
the task and the monitor, as perfectly expected by me, remains off without
waking up unless i move the mouse or i use the keyboard.
Instead in Vista when i perform the same operation on the task scheduler the
computer wakes up at the specified hour performing the task but waking up
also the monitor that displays a black screen and the mouse pointer and it
remains in this state until the task ends.
This is not logic, i want the monitor to completely remains in standby
exactly as it happens in Xp, unless it's me who uses the mouse or the
keyboard. Why waking up the monitor not saving power only to display a black
screen with a mouse pointer??
Is this something related to my settings or it's a general Vista bug, is
there a way to correct it?
I am using a new hp computer and a new monitor so they are all clearly
compatible with all the last ACPI standards, for the monitor connection i am
using the DVI interface. I have the same problem on my Vista laptop.
Thanks a lot for reading me and trying to help me.
Best regards
Michele