Problem adusting screen resolution

I have a Dell Dimensions 2400 desktop. It originally came with Windows Service Pack 1 which I have just reinstalled and upgraded to Service pack 3.

In display properties, I cant move the screen resolution slider for the monitor to stop between 800 x 600 and 1024x 628. It goes from one end to the other but wont stop inbetween .When I use another monitor its fine and I can use the slider properly.

The monitor is a Dell E152FP and came with the computer. I have reinstalled the driver from the monitor's cd installation disk but it still doesnt work. Please help