Solved using cd key twice

would be possible to reuse my dream spark windows 8 CD key on another computer to upgrade it to media center.

NO.

One key=one install= one PC.

yes but the isnt the old key replaced when you install media center.

Hello Dragon,

Sorry, but no. Only one key per activation.

The Windows 8 Media Center Pack doesn't replace your Windows 8 key, it's just an added feature that gets unlocked using the Media Center key.

You can still get the Windows 8 Media Center Pack for free until January 31, 2013 if you have Windows 8 Pro installed on the other computer though. You will just need to use a different email address to get another free product key.

See:
http://www.eightforums.com/tutorials/6559-windows-media-center-add-windows-8-a.html

and

Add features - Microsoft Windows


Hope this helps,
Shawn