Offer valid from October 26, 2012 until January 31, 2013 and is limited to five upgrade licenses per customer. To install Windows 8 Pro, customers must be running Windows XP SP3, Windows Vista, or Windows 7. Get the full details on the special offer.
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If you only take the digital download, you can later choose to write it onto a DVD yourself or save the .iso image (~2.7 GB), store it away and write to a USB flash whenever you need to using Windows 7 USD/DVD Download Tool. You can boot the DVD/USB flash for a clean install, or if you prefer the upgrade path, from the old Windows run the X:\setup.exe and follow the steps.