W Wood

When I stop using computerit will go into stanby and screen and hard dsrive will shut down but later when I want to use it I have a Flashing icon in the upper left corner of screen. There seems to be no way start the computer so have to power off and restart. It used to power down but would come back to life when pressing the mouse button. How do I get rid of this problem.?
System is an old aptiva running windows xp.
Thank You.
Woody 97