Hal, I'm no expert at computers but my easy fix for playing sacred underworld in full screen with no issues, and I tell this to everyone, was to save an old HD/partition a new HD, with win XP32, solely for older games that are no longer updated. I also have the original cd's not steam or download. For whatever reason I never had any issue with neither of the sacred games or their expansions like other people do.
Unless thats not probable or viable for you. Just my experience.