A whole lot of Windows Vista beta testers are miffed to find out that Vista’s Media Center stopped working properly on December 31. The problem: The MPEG 2 decoder used in Release Candidate 1 was only licensed through the end of the year, so, when the date came, poof! No more Live TV. Microsoft fixed the issue in RC2 (presumably by licensing the decoder through the actual end of the beta period), but many have not been able to upgrade to RC2, due to various availability and upgrade path issues.
For now, the best bet is to downgrade to Windows XP Media Center Edition, or, if you never had that version, find a way to upgrade to RC2 (you can find it on Bit Torrent, and the product keys should work). Installing a third-party MPEG2 decoder won’t help. Otherwise, hold your breath and live without fully using Media Center for four more weeks (yes, I know it won’t be easy) and pick up Vista Home Premium when you can.
If you’re interested, you can pre-order an upgrade copy of Windows Vista Ultimate signed by Bill Gates himself. It’s extremely limited, with Big Bill signing only 20,000 copies, and as TDavid notes, it is likely the last operating system Gates will be involved in and thus autograph.