Just to add: The XP Media Center operating system is only available as a pre-installation on specific computers. There is no retail version of this software.
The Media Center program in Vista is unique to Vista.
Windows Media Player 11 is available for both XP and Vista.
You could add XP as a dual boot option along side Vista. This also is not as easy as it appears as XP will over write the boot sector. There is a brief guide here:
http://apcmag.com/5485/dualbooting_vista_and_xp
If you plan to remove Vista and replace with XP you will need to format the hard drive outside of Windows as Windows will not allow itself to be downgraded.
You probably also will void all the warranties on your computer.
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