desktop is sideways???

I have a system running windows xp home. Somehow, the whole desktop is completely sideways and it is very hard to run the mouse, almost like it has been altered as well. I have looked at all the properties and settings and can not see anything that would turn it upright again. I have been in the computer service business for ten years and have never seen anything like this. When the system boots the windows xp logo and bios info is upright, but when the desktop comes up it is sideways. I hope someone can help me with this problem.