You can try that but some monitors store their settings. You might want to go into the monitor setup screen (there should be a button on the monitor that says menu) then look for screen geometry or somthing like that.
If you send me the model of monitor you are using I could try and lookup the user guide for it.
Could also be in the video card settings. Right click on a clear area on the desktop and click properties. Screen rotation stetting could be in there.