You can add Windows Media Center for free for a limited time to a Windows 8 Pro installation by using the offer on the Microsoft Windows site, under Add Features.
You can follow these links:
You need to be quick, this offer is only valid until January 31, 2013. You can use multiple email addresses to get multiple keys, but note that you are only legally allowed to claim 5 keys.
Note also that the key must be activated no later than January 31, 2013. So this means that you need to claim the key, and also install and activate the key on your Widows 8 Pro PC before this date.
This raises an interesting question – what happens if you then need to format your PC, do you now need to buy the Windows 8 Media Center Pack?
Once updated, you can find media center on the start screen. Also interesting to see that it updates the OS version in the System properties page.