On further review all these products appear as part of the Operating System.
Dell and HP offer their own versions of XP MCE.
You can also find OEM disks for sale although Microsoft never released this as a retail OS.
From what I read the XP MCE features are included in Vista. At least in some versions.
Here are some links to reviews:
http://www.microsoft.com/windowsxp/m...r/default.mspx
http://www.winsupersite.com/reviews/...xp_mce2005.asp
http://www.gizmocafe.com/tv-video/wi...ia-center.aspx
http://reviews.cnet.com/windows/micr...-31138402.html