Nvidia cards using driver version 256 and above currently have issues with the viewport in Houdini 11. A workaround has been added to Houdini 11.0.458, and the next version of the production build (due out early next week).

If it is not possible for you to upgrade Houdini, you can downgrade your Nvidia driver to version 197 or less (to version 185).

Apologies for the inconvenience.