It took me a while to figure it out because I use resolution 800X600 and the Hot Key won't work in that Resolution.
Right Click Desktop - NVIEW Properties - Hot Keys - Click the plus sign in front of - Display Mode - Click Rotate Display - Add - At the top Select
90 degrees - Left click None - Hold down on the Alt Key and press R
Click OK
Your Resolution needs to be set to 1024 X 768 for your Hot Key to work.
I used the Alt key + R for Rotate. Easy to remember.
When you open NVIDIA Display you will see the Hot Key listed for 90 degrees.
I don't think a restart is needed.
Anyway the above worked for me when I held down on the Alt Key and Pressed R
One other thing. If NVIEW Properties is not showing than a setting has been changed. I changed it once but right this minute I can't give directions to find this setting. I will if needed.