since it is legit they can't provide support.
Sorry, I meant because I didn't need a new product key (because it's valid/legit) they couldn't provide me with a new one, but then again there's no word that key would work.
I phoned up Microsoft again, seeking installation help. I got put through to some US call-center and explained the situation; he said that I'd either need the company's credit-card or support contract details, otherwise they can't provide installation technical support.
This really is getting effing silly. Installation support dealing with activation should be free on account of this being Microsoft's fault.