The Spectre has an aluminum chassis, carbon fiber bottom and weighs just 2.45 pounds, while the Envy machines come in two screen sizes.   HP has unveiled its four latest premium laptop computers: the HP Spectre, which is the company's thinnest laptop; a 2-in-1 convertible Envy Hybrid; and two other Envy models with 15.6- or 17.3-inch displays. The...