Pixel format not accelerated indicates that you need to ensure you have a proper display driver installed. Windows 7 (and Steam) often install a generic driver from Microsoft that just doesn't cut it for this game.
Go to the website of your specific video card manufacturer (AMD) and download the latest driver for your card directly from them. If you receive any prompts that you need to use a manufacturer-specific driver, well . . . we'll try to work through it by forcing a generic, up to date, driver.
Just be sure to turn your computer off and back on again after installing the new driver, otherwise you'll get the same error.