AFAIK, you'll need to create either and DVD or an bootable USB Drive with your ISO. You may not boot from that Media, you could start the setup from the installed Windows 8. Let it do the Updates as asked in the first scrren and use the PKey you posted for to install. Chose Update at the scrren which gives you the choice of upgrade or clean install. Now you'll get an messages what all could be taken over. If it's OK for you, just carry on, if not stop the setup at that moment without to lose anything. After installing change the PK as advice on this forum to your lehit key an activate. Than go ahead with the Media Center PK and setup.
Just upgraded to 8.1 from 8.1 preview. Activated using my legit key. First Activate using 8 key then add feature and insert ur MC key.
Was the preview version 8.1 with WMC already or did you just add WMC after upgrading? What ISO did you use?