> I suffered this with a 2nd hand Windows 7 computer my mother was using that kept complaining about being counterfeit.

That has been known to happen [1] (it's never happened to me personally). I suspect it has something to do with buggy AV or anti-malware software.

> For nearly 1 hour I tried to find a way to buy that Windows 7 licence key and found absolutely no way to do it.

You don't have a Best Buy type store near you?

[1] http://www.infoworld.com/article/2859267/operating-systems/w...