You will be able to tell when you begin the installation whether or not your Media Center DVD has the Windows operating system on it.  As I stated, you will boot the DVD and use it to set up the partitions for the operating system and the Media Center files.  The on screen directions should tell you if you need to remove the DVD and boot the Windows XP Pro set-up disk.  If you do not need to perform that step, you will find a step that starts Windows XP set-up while the DVD is still in the drive.
I have Dimension 8400. I have to install new HD. Can I do CleanInstall with Dell Reinstallation DVD Microsoft window XP Media Center Edition2005? Is all window XP Pro OS on this DVD? How come the Retail MCE 2005 has twoCD?
Dell Reinstallation DVD MCE 2005