Read http://support.apple.com/kb/TS4135?viewlocale=en_US and try what they offer there.
I don't know how else to describe it.... several times when I have been away from my MacBook Pro and awakened it from "sleep," my cursor is at the corner of a gray/brown box that contains the area for my password. When I move the cursor, the box enlarges, or rather creates a trail of little boxes in the same color. It would end up filling the entire screen I believe if I continued. The only cure I've found is to press the power button to turn it off. Any idea what is causing this? As usual, thanks.....
Old Granny Lola