In my experience this usually happens on Intel Graphics Cards.
Go to Control Panel, Display, Settings then Advanced. You should have something called Intel Display Properties. There is an option there to change the Rotation (its prob at 90 degrees). Once its back to Normal disable rotation by uncehecking the tick box.
A possibly shorter way to do this is double click the intel graphics settings in your taskbar, should be beside the clock. Just double click it and the rotation option is there.
Phil Roberts, July 2009
