For a limited time, Microsoft is offering the Media Center add-on for Windows 8 Pro as a free option. At least according to the information on Microsoft's website located here.
They are offering this from October 26th 2012 through January 31st 2013, limited to one key per email address. You have to activate the product key by January 31st 2013 as well. The regular pricing on this add-on is $9.99. It is likely that if you use this link today, that the key won't be emailed until tomorrow. What isn't clear at this time is that if you must activate before January 31st 2013, what happens if you rebuild and reactivate your system after this point? I'm sure details on this will follow.
Note that if you try to do the upgrade from the Control Panel in Windows 8 they will charge you the full $9.99 fee, so be sure to use the web link above.
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