It could be a number of things, none of them very pleasant, including the imminent failure of the motherboard, the imminent failure of the graphics chip or, possibly, a developing short in the cable that connects the screen to the motherboard.
I did a quick google and the Dell latitude D630 has reported screen problems. Do you know if the GPU (the graphics processor) is integrated or if it runs an nVidia graphics card? If you right-click on the desktop a context menu should come up with the option to adjust the graphics properties; if the system has integrated graphics it'll say something about Intel graphics; if it has an nVidia graphics card it'll say something about adjusting the nVidia graphics properties.