Carlo fixed the problem on his PC. He did it by running TuneUp Utilities ---> Disk Doctor. As far as I know, Disk Doctor is just a ChkDsk command on Windows with a little bit more features. And it just corrects errors on the hard disk. What could possibly be on the hard drive that can cause problems on the monitor? I tried reinstalling the drivers, to no avail.
The problem on his PC was that vertical lines appear on the monitor across the whole screen. At first I thought the problem was the monitor. But I tried switching his and JJ's monitors. Same thing. Then I thought, "it was the video card". I dismissed this theory easily. I don't think video cards can get damaged that easily even after years of usage. Then finally, I thought the culprit was the motherboard. I just knew it was the problem.
The whole point of the story is that I'm still not convinced that the hard drive was the problem.