Final Cut is suddenly asking for my key...

I had already had Final Cut Express 4 installed for a while as I got it about a year ago. And because im so unorganised I lost some bits and bobs from it. I have the box, and the CD still, but a while ago it started asking me for a key, I guess this was on a peice of paper that I lost. Is there ANY way of re-validating it? I have the box and disk as I said, but I can't load up the programme any more because I don't have the key any more. And why did it start asking for it when I had already input it?

Thanks