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.

Call them (MS) and they will register it.
The registration doesn't have a limit.

Just call customer service and they'll fix it? Do they give me a new key or just set it so the old one will work again?

Tough to say, call and it'll be straightened out.
It has been for anyone that calls with a legitimate problem like yours.