One more option if you don't want or don't have the necessary privileges to modify a registry key: Open PowerPoint, then when you select the ppt/pptx file to open, click on the little arrow on the button "Open" and there should be an option to open the file as read-only. I know this doesn't answer your question, as it's not the shift+right click menu, but wanted to include it for others who may be more comfortable with this option.