Problem is with a Dell (about two years old) driving a touch-screen monitor using Intel Extreme graphics (which I believe is on the motherboard). XP Pro SP1, though this is irrelevant. Machine is on a small network, which is not internet connected. No software changes for ages.
The monitor can have just two resolutions, 800 x 600 (which is the normal one wanted) and 1024 x 768, four possible screen refresh rates, and two colour depths.
"Suddenly" the 800 x 600 resolution displays jagged lines through characters/graphics (the Windows startup logo as well), which cannot be got rid of by changing any of the parameters mentioned above. The text and graphics are readable, but distorted. Using the 1024 x 768 resolution displays perfectly!
All the usual "bright ideas" about changing drivers, etc, aren't applicable because of the "nothing's been touched" nature of the environment. It wasn't possible to do this at the time, but today I hope someone will be trying another, working perfectly, touch-screen monitor instead of the problematic one. But my theory is that the graphics card/controller/feature on the motherboard is not working correctly, which means a new motherboard, which probably means it would be cheaper to replace the PC entirely!
Any agreement/disagreement? Have I missed anything?