Registration used too many times

I have an issue... I bought FSX on disc a few months ago then installed and registered it. A short while later I had to do a low-level format on my computer, reinstalled and re-registered it. A few days ago I formatted my computer again, reinstalled but can't register. I am being told I have used the key too many times. Now there is a problem, the game only comes with one registration key(obviously). How am I supposed to fix this, or how do obtain a new one free? I really don't want to buy a new disc when the one I've got is still in mint condition. Thanks in advance to anyone willing and able to help.