Within Windows XP, certain video drivers or graphics card chipsets supporting 180 degree screen rotation can cause the Windows XP display to be presented upside down. This can be (but is not always) tied to lowering the screen resolution in some accessibility programs.
To revert to a right-side up display, hold down the CTRL, ALT and UP (arrow) keys.
Another option is to contact Microsoft for a fix. See the Microsoft Knowledge Base for more information.