Right Click on a blank piece of your desktop, in that menu select "Graphics Options" > "Graphics Properties.

That will open a Pop-up Menu, you'll need the 5th tab which says "Rotation" and select the option you want from there.

You can alos try holding down + + Any cursor key and that will rotate your desktop too (Up = Upside down, Down = Normal, Right = 90 deg clockwise and Left = 90 deg anti-clockwise)

* Remember, some graphic drivers will not support this.