Hi!
I discussed your problem with a live Microsoft expert and you can see the discussion below in the source section for this answer. To summarize, if you want to use Windows 8 Pro, you have to install XP, Vista, or Windows 7 and then upgrade using your authentication key. To buy it on line use this link http://windows.microsoft.com/en-us/windows/buy?ocid=GA8_O_WOL_Hero_ShopHP_FPP_Null
Microsoft does not sell Windows 8 Pro. They sell only upgrade version for which you must have already installed a legal version of XP, Vista, or Windows 7.
Thank you for choosing Microsoft Store. An Answer Tech will be with you shortly.
Privacy Statement
You are now chatting with 'Peter'.
Peter: Thanks for contacting Microsoft Store Chat, my name is Peter! How can I help you today?
You: I have an authentication key that is valid for upgrading from windows 7 to windows 8 Pro but my laptop is new and has no operating system. Can you give me a link to download windows 8 Pro from and use my authentication key that I already have?
Peter: Id be more than happy to help you with Windows today!
Peter: I am terribly sorry but unfortunately ther eis no open download link.
Peter: Additionally you cannot use an upgrade as a full install, it will not work.
You: The guy from whom I bought the computer installed windows 8 Pro on it but the installation key that he gave me is only for update from update for an earlier version so it won't work. What solution do you propose?
Peter: The only solution is to install XP, Vista, or 7 and then upgrade.
You: But I don't have an older version of windows with me. Can I use a 3o day trial version of Windows 7 to upgrade to windows 8 Pro and use my upgrade installation key?
Peter: Ther eare no Windows trials.
You: How much would it cost just to buy a new Windows 8 Pro key while I have it installed on my computer already?
Peter: We only offer the upgrades at this time.
You: So the only option that I have is to have an older verion of operating system from Microsoft and then upgrade using my windows 8 Pro key?
Peter: Correct.
You: ok. Thank you very much.
Peter: You're welcome!
Peter: Is there anything else I can assist you with today?
You: Can I buy a windows xp on line? If yes, then what is the price?
Peter: No we do not sell that product.
You: ok. Thanks for your hlep.
Peter: You're welcome!
Peter: Thanks again for chatting with us at the Microsoft Store, we appreciate your business!
Thank you for chatting with us. Please click the "Close" button on the top right of the chat window to tell us how we did today.