Screen cannot power down

I'm not sure if this is the right section to ask this question, but here
goes. I just recently upgraded from XP to Vista HP. One of the problems I'm
having is that the screen will not power down (or shut off) like it used to
in XP. I checked my power maintenance settings and the screen is set to turn
off after 20 minutes. The screen saver turns on after 10 minutes, but 10
minutes later (when the screen should turn off) the screen saver stops, just
as if someone had touched the mouse or keyboard. Does anyone have any ideas
about what could be happening?