Sounds like your video card. Many video cards have the ability to rotate the image on the screen.
The steps to correct this vary from card to card, but I'll try to give you a general idea of where to look.
Right click on the desktop and select "Properties" from the context menu. Now select the "Settings" tab from the Window that opens. Click on the "Advanced" button and look for something that sounds like it may be connected to rotating the screen (like "rotation") and change the setting to normal. If no such setting is show, see if there is a tab that has your graphics card's name on it (see illustration)
Search there for somethinglike "Change Rotation":