Lets consider this - maidenlewes has said that the "shop bought" copy of XP "was on my old PC," not is on it. So one problem out of the way.
But if this version of XP was supplied OEM with the old machine, then it doesn't matter whether it's on it or not - you cannot use it on another machine. If it's a retail version, and it's no longer on the old machine, then you have no problem. Go ahead and install it on the on the media centre machine.
I am puzzled by this. What do you not like about MCE? I have Media Center 2005 on my PC and unless I click on the Media Center shortcut then it runs as if it were an XP Pro machine. After all the Media Center bits are only add ons to XP.
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