Mouse Erasing My Screen

Help! When my Screensaver comes on after several minutes the screen turns black; when I attempt to wake my iMac up by pressing esc nothing happens. Then when I move the mouse it erases the black screen. Where ever I move the mouse it just erases the black to expose the log on screen.

Thanks

And you have a screensaver ... why exactly?

Now that you ask I'm not sure; I just always used one. Do you ask because you think it has something to do with the problem?