My mother-in-law just called about an issue where her computer screen was displayed completely upside-down. It was not just the desktop image, but also the icons (and everything). She felt like it happened when she was logging out.
Turns out that Windows XP has a shortcut combination that does this – Ctrl + Alt + Up.
It is crazy that this combination is so close to the Ctrl + Alt + Del that some use to log out, use Task Manager, etc.