Why Does Personal Screen Revolve 90 Degrees?

Every now then , I go to use my computer and the screen image has turned 90 degrees. Everything next to the screen works, but it's rather difficult to convey the mouse around because of the changed orientation. Whether I do a system restore, it fixes the problem. But I'm concerned about WHY it does it, and if there's a way to stop it from happening in the future. My computer is a Dell Inspiron E1505 laptop near Windows XP Media Operating System.