A: To add features to Windows 8 through additional Windows packs such as the Windows 8 Media Center Pack or the Windows 8 Pro Pack, use the Windows Activation area of the Action Center Control Panel applet. Click the Add features to your edition button, and you will be prompted for the product key for the additional features (see screen shots). Enter the key, accept any license agreements relating to the new feature, and then the new feature will be added.

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Adding Features to Windows 8 Using the Windows Activation Area

Learn more from "Q: How do I add and remove features from Windows 7?"