The noninstrument-rated pilot's improper decision to continue visual flight rules flight into instrument meteorological conditions and his subsequent spatial disorientation. Contributing to the accident was the copilot's improper evaluation of the weather conditions after receiving several weather briefings for the flight. Also contributing to the accident was the pilot-in-command's cognitive and psychomotor impairment due to recent use of an overthecounter sedating antihistamine and the pilots' personal pressure to get home.