Microsoft has finally added operating-system level support for viewing raw camera files. Yay! You'd think that once you've downloaded and installed Microsoft's Camera Codec Pack you'd immediately start to see those lovely image thumbnails. You don't. Boo! Though it's easy to get the thumbnails to render, it's not obvious how to do so.
First, make sure your camera's raw files are supported by checking the list on the download page. Unfortunately, some of the most recent popular models are missing, such as the Nikon D3100 and D5100, Canon EOS 60D, all of the Olympus PEN series, and the most recent generation of Panasonic G series. If you're lucky and your model is supported, and you're running Vista or Windows 7, download and install the codec.
Once you've installed the codec pack, you should be able to preview any of the images. However, you'll still see the generic thumbnails. So you're going to have to force thumbnail creation. In order to do that, you have to open at least one of the photos in Windows Live Photo Gallery. To do that, right-click on one of the images and select Preview within the Preview display, choose the Edit, organize, or share menu option.
When you open the image in the Gallery, it automatically begins to render thumbnails for all of the images in the folder.
After Gallery has completed generating the thumbnails, you can go back to Windows Explorer and see that all your thumbnails are now there.
As far as I can tell, you have to do this every time you copy images to your hard disk. It's a pretty inelegant way to handle the files and it's disappointing how many current cameras are missing. The codecs don't seem to do a great job rendering, and they lack built-in distortion correction for cameras that incorporate it (like Panasonic's). Plus, I doubt Microsoft is going to be updating the codec pack as frequently as is necessary to keep up with the rapid release cycle of new enthusiast cameras, each which comes with a new version of the format. Still, it's there. Yay.