A little clarification:
First, I should say I'm not a huge Microsoft booster, I just built an
MCE machine cause it was cheap and did what I want.
Windows Media Center is just Windows XP PRO with a slightly different key so that Windows Update
knows to give your MCE updates as well. It can be purchased
bundled with a computer, or downloaded
from Microsoft assuming you have an MSDN subscription.
MCE is Microsoft's competitor to Tivo or Replay in the embedded market,
or Snapstream and other PVR programs found in the Windows world.
It uses the same Electronic Program Guide that most other Windows PVRs used called titantv http://www.titantv.com
to get program listings.
From the posts on do-it-yourself websites I'd say the majority of MCE machines are self-built.
Mine cost $255 to build, for a guide check out http://www.thegreenbutton.com or http://www.machineroom.org/hometheater/archive/000389.html
The big downside to MCE boxes? They record in their own format meant to wrap the content
and protect the copyrights of the content owner, however their
format is just MPEG with a wrapper.
So, there are directshow graphs and other ways to get at the files if you want to, exporting the
content and editing it is not too hard.
Again, check out the green button for details.