I did something to my Vostro 1310 laptop and now everything on the screen is upside down.
I have a Vostro 1500 with NVidia DIsplay drivers installed and use a simular option to rotate the displayed information prior of rotating my external screen (nice to surf through google or to view whole pages with Word).
(I am a german user, so I can only try to translate into the right terms) Right-click on the desktop and choose NVIDIA Sytemtray. There you have the option "rotate screen" or something simular. There you can choose between four options of rotation including not rotated view. Try this and a not rotated windows should appear on the screen.
Does this help you?