Sorry for the wait, but I think I've found the problem. In XP, if I change the resolution to 1200x1600 (which I did just to test), I'm guessing the monitor sends a "hell no!" to the graphics card, which stores that the monitor can't handle big resolutions. But the only big resolution it can't handle (that is an option) is 1200x1600.
I'll try to fix it by uninstalling the graphics card driver and reinstalling it.