The Sierra Help Pages, is a site by Collector, offering solutions and support for old Sierra games so they can be played on new/modern systems.
Among other things, it includes instructions and new, improved installers that solve most of the in-game issues for the Quest for Glory series. Before you look into any other solution, it is highly recommended that you use Collector's installers to install the Quest for Glory games, therefore avoiding most of the trouble with just one simple step.
You can download the latest Quest for Glory installers here.