People who were testing the RC1 copy of Windows Vista, the most distributed of all the betas, got a nasty holiday surprise on new years eve. Windows Media Center uses MPEG 2 decoder and Dolby Digital components, which are definitely not free. Microsoft pays the respective owners of the decoders for a license, which expired for Windows Vista RC1 users on December 31st. Microsoft cannot renew the license, and has declined to say whether or not the license could have been extended. The rest of Windows Vista RC1 will still work until June 2007.
Source: CNET