How to change from landscape to portrait on flat screen monitor that turns 90 de

I read this at another site.

"Some graphics drivers, such as NVidia's, include an option to rotate the
screen for use with a flat panel monitor that can be rotated.

To see if the problem lies here, right click on your desktop and select
properties. Click on the Settings tab, then the Advanced button.
One of the tabs will be the name of your graphics card.

Look through the options there. If it is an NVidia card you're using, it
will be under NVRotate."

Hope it works for you.