First thing to check, is if your video card drivers are up-to-date, if not update the drivers. Also just to make sure check if Rhino is up-to-date: Go to Help -> Check for Updates... clock the text check now to find if you have the latest version.
What videocard do you have on what Windows version?
The videocard info can be found here:
Does the problem of only partial updating viewports happen always or in special cases?