Which is the fastest way to get genuine?

Hi I am using Windows 7 Home Premium 64 bit. I was sure it is genuine, I had it from my family home, but It is meant for one pc only. So it is not working now - it is not genuine. I want to get genuine - and my question is - which is the cheapest and fastest way to do it? I do not need Windows for more computers than one, and I do not need to transfer it to other pc in the future. Is...