If you bought the key on Ebay, you have 45 days from the date of purchase in order to get a refund. It's the only way Microsoft will handle it. Get in touch with Microsoft, and Microsoft will get in touch with whomever sold you the key. That error code usually indicates a blocked MSDN key, where a seller sells you a key for Windows, and continues to sell the same key to as many people as he can before getting caught.