New Win7 key

A few months ago I built a new pc and took the hard drive out of my old one and put it in the new. Completely different hardware but surprisingly, Windows didn't complain. Anyway, the next day after turning the pc on, I got a msg from windows about activation and that there was a problem. It gave me a phone # to call and all I had to do was type in my key and I was given a new one. My question is, if I reformat one day, will I have have a problem when I have to type in the key during installation? I used a key finder and it showed I still had the original one. I wrote down the new key but misplaced it.