Screen Area Resulations

I am unable to adjust the screen area due to the fact that the slide to make this adustment will not move. I am using windows 98se.on a E-machine model 366 and have increased the RAM from 32 to 130. Thank you for any help you may be able to give meas I have exausted all efforts to correct the problem. The screen is now set at 640 by 480 pixels.