If you enable highlighting, the box the end user has is selected by default and you cannot change that. If you want that option added in a future version, submit a feature request.
See www.grainge.org for RoboHelp and Authoring tips
See this thread - Re: Changing the default selected setting on the "Highlight search results" field in Search - for a "home-rolled" solution. It involves editing the whform.js file to remove a "checked" status.