It was annoying that XP had no idea that, say, a CR2 file or NEF file was an image. Vista attempts to solve that problem, as well as the problem of not-yet invented, potentially unrecognizable file types with its Windows Imaging Components system. The good news: Anyone (such as a camera manufacturer) that makes a file type can also make a little piece of software, called a codec, that explains that file type to the OS. The less-good news: Since the camera-makers are responsible for keeping their codecs on their websites, Windows Photo Gallery may or may not lead you directly your codec's download site. If it doesn't work, chances are a bit of Googling will turn the codec up.
Once you get it installed, you'll be able to see unadjustable previews of your RAW file in Windows Photo Gallery and throughout the operating system at large.