I have an HP Pavilion running Windows XP. Whenever I put the computer on stand-by mode it doesn't stay there. The screen will remain dark, but I can hear it constantly "coming to life" as it does when you move the mouse to wake it up. It will click off, but then almost instantly start up again. Nothing appears on the screen until I actually do jiggle the mouse - but the constant on and off noises are really annoying. What's going on?