this can be caused a few different ways. First lets see if this works here.
Right click on your desktop and click on properties.
In the properties you will see a tab called "screen saver" click on it
In "screen saver" you will see a display called "monitor power" click on "power"
this will bring another screen up which says "power option properties"
You will see where it says "settings for Energy Star power scheme"
their is a place for:
turn off monitor
turn off hard disk
you can adjust those setting to whichever way you want. once you have adjusted the settings make sure and click "APPLY"
This should fix the problem. If not let me know.
Please click the Green Accept Button.