I think that what Didou was getting as it there have been reports that Microsoft will give you a valid Copy of Windows if you bought one that wasn't legal.
Although haven't heard too much of them actually handing these things out just Microsoft saying they will if you tell them where you got it, and all the stuff I remember seeing about it mostly delt with if you went down to a local computer shop and bought a new computer. Then instead of giving you a legal copy of windows the shop put something on there that would word and saved them money. So I don't know if they will do anyting with your situation with buying it off ebay, but it is really your only option right now.