Hewlett-Packard unveiled a Windows-powered touchscreen tablet computer on Friday aimed at professionals, the latest entry into a growing market dominated by Apple's iPad.The "HP Slate 500" which features an 8.9-inch (22.6-centimeter) screen, slightly smaller than the iPad's 9.7-inch (24.6-cm) display, costs 800 dollars, about the same price as the top-of-the-line ...