Microsoft Media Center Drops Stations Making Early Transition to Digital
Some users of pre-TV Pack Windows Media Center software lost guide data for stations that recently shut off their analog channels. Stations that ceased analog broadcasting showed up as "no data available" in the channel guide. Microsoft said this was because in Windows Vista, the guide data was obtained from two sources—FCC data and a third-party company that provided analog broadcasting data. In the pre-TV Pack versions of Windows Media Center, Microsoft mapped ATSC channels to stations' corresponding analog channel. When the analog channel was shut down, this guide data was disappeared.
Microsoft should have resolved the issue by now. For additional information, see Microsoft restores guide data for ATSC channels in Media Center
and this forum thread from thegreenbutton.com