If you've got a Media Center PC running the first version of Media Center and didn't get an upgrade to Media Center 2004 then don't worry. When you install Service Pack 2 for Windows XP you'll be upgraded to Media Center 2004.
See here for more info.
If you want to get an early look at Service Pack 2 you can do so here. Be warned though that's a preview version of Service Pack 2, if you're not comfortable with installing pre-release software then wait for the final release later.
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