The key for XP Home would be the one for XP Home and not the Media Center one. It's that simple. These are not interchangeable.

Now before you install XP Home be sure you have a XP SP2 Home CD. The reason I insist on that is to avoid install trouble.

-> Now you need an INSTALL PLAN. Read http://forums.cnet.com/5208-7587_102-0.html?forumID=69&threadID=271360&messageID=2699006#2699006 to see an example plan.

Remember all the files on the PC will go away.

I never heard of Microsoft offering a "media center pack." There are versions of XP called Media Center but let's skip that discussion.

The Vista 7 is beta, will expire and frankly would only be a desperate man's solution.
Bob