Screen goes black suddenly

I've been having a problem with my 1501 Inspirton laptop - suddenly my screen will go black, sometimes with a little box with lines on it in the bottom part of the screen; at these times the computer freezes up and I have to turn the computer off using the power button and reboot again.  Does anyone know what this could be?  It always starts back up again but it has been happening more lately.  My laptop is 2 years old.