Vista Media Center does an excellent job of scanning and finding ATSC channels using a broadcast antenna with an ATSC tuner. However, it does not add sub-channels to the guide and the user cannot access channels not in the guide.I found the following procedure will enable you to add the subchannels after creating the guide using the Media Center setup.
Go to www.zap2it.com. Under
find it fast near the top just right of center, type in your zipcode and select
tv listings.
Select antenna under local broadcast on the bottom. For example in Reston type
in a zip code of 20191 and select antenna.
Scroll down and locate your local digital tv subchannel stations. These stations will have the letters DT at the end of their call sign. For instance, WRC (channel 4) will appear later in the listing as WRCDT (channel 4.1) This should already appear in your guide that is created by Vista. Next you will se a sub-channel WRCDT2 (channel 4.2). You will need to add this channel manually. WJLA (channel 7 has a sub-channel as does WUSA (channel 9) and WETA (channel 26).The point is there are a number of stations that have sub-channels that you can add. It is suggested that you print the channel listing from zap2it.
Next, go to Media Center Edition and select tasks, settings, tv, guide, add missing channels, add missing dtv channels. To enter the channel 4 sub-channel type 04 in the left entry block. The digit 2 should appear in the right block and the call sign WRCDT2 will appear. Click add channel to add that channel. This will take you back to the previous entry point where you can select add missing dtv channels if you have more or done if you are finished. When done press Control G to go to the guide to look at your entries. To access channel 4-2 enter 1042 in the numeric keypad. Note that although the guide will list the sub-channel and let you access it the listing will probably be incorrect unless you have selected the local broadcast listing while performing you setup. If you are also connecting to cable and select that option in setup then the guide listings will be incorrect for the sub-channels. See Program guide listings in Windows Vista Media Center for additional information on this topic.
Hopefully, in the future Media Center will locate the sub-channels automaticallly. Regardless, now you can do so manually.
Thanks to the site http://mcerob.notlong.com for much of the information given in this document. I regret that I do not have the resources to provide detailed help in implementing the above procedure.