Here is another method that worked for me (because I couldn't find the Reader thing anywhere). Right-click on the desktop, and there is a menu item "Graphics Properties..." (not the menu "Graphics Options").
It takes up to 30 seconds to open, so it takes patience. When it opens, the default tab in the left frame is probably "Display Devices". Select the tab "Display Settings" and there is an area titled "Rotation", and a check box labeled "Enable Rotation".
Make sure that is unchecked, and the problem should be solved.