You don’t need to use your mouse to enable Google’s visual previews for search results. Just press the right arrow key to show a small screenshot for the currently selected search result. You can navigate using the up/down arrow keys to highlight a different result and use the left arrow key to hide the previews.
Google Instant Previews can’t be disabled from the settings page and, despite having a similar name, it’s not related to Google Instant. If you disable Google Instant, you’ll still see the magnifying glasses next to the search results, but the keyboard shortcuts are no longer available.
This feature is not yet available to everyone, but you can try it using this special page or by adding &esrch=instantpreviews to the URL of a search results page.