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Windows Product Key doesn't work on other Windows

My situation is as follows: I have three license keys for Windows:

  • (A) 1 Windows 7 Professional
  • (B) 1 "Microsoft Windows 7 Professional with Service Pack 1 32/64-bit (German) - DreamSpark"
  • (C) 1 Windows 7 Home Prem OA

Now I had a computer that ran with my (A) license for some years, anyway last month I bought a new laptop which I will use instead of the computer. The laptop came with license key (C), but since I didn't want all programs and the stuff that comes with a new Windows on it, I formatted it, using my (A) product key.

In hindsight, that was not such a brilliant idea as I now see, because apparently I cannot use the older license (A) for the activation of my new laptop, since it is already connected to the old PC.

But I have got license (B) from my university which offered a free license for every student, I requested (B) about one month ago. Nobody ever used it and I didn't share it with anyone. Now it appears as if it is not compatible with my current version of windows on my laptop, although both are Windows 7 Professional. When I try to activate my product by changing the laptops' product key to (B), I get the error message that this key is not a valid key DESPITE the fact, that I copy-pasted it from the Dreamspark page where I got it from.

So there is no possibility of misspelling or anything, what could be the reason for the problems I have?

EDIT: I just want to get my laptop run with a valid product key without re-formatting it. If there is a possibility to get (C) running via an upgrade or something, I really don't care which Windows7 I will use. Fact is, that I have 3 product keys, but the laptop is not activated, which I want to change.