The free Microsoft Camera Codec Pack for Windows Live Photo Gallery 2011 plugs a gap previously filled by third-party software such as Axel Rietschin's FastPictureViewer Codec Pack, allowing Windows to create thumbnails for raw files in Windows Explorer. The Microsoft codec pack supports 108 cameras from Canon, Epson, Leica, Minolta, Nikon, Olympus, Panasonic, Pentax, and Sony.
The list of supported models is a bit dated, however, with many recent, popular models such as the Canon EOS 60D, Nikon D7000, Pentax K-5, Sony NEX-series, and all but one Olympus PEN model (among others) notable by their absence. For Pentax shooters, Microsoft also doesn't support the open DNG raw format, but instead accepts only the proprietary PEF format. There's no question that the FastPictureViewer pack offers much more comprehensive support, with somewhere shy of 400 models being compatible. Still, for photographers shooting with supported models, the ability to preview their images in Windows Explorer and process photos using Windows' built-in photo gallery application--all free of charge--is a great addition.
More details in the Windows Live blog post.