Problems with Product Key

I need to get my microsoft excel running i no longer have the cd's, and it's asking for the 25-character product key. I installed magical jelly bean to find my product key, but the key it gave me was no help. The product key on the computer itself isn't the correct product key either.

Btw here's some info just incase you might have some idea's.

Microsoft office 2003, it's an hp computer with the product key on the side saying Microsoft xp media center 2005. Not sure if i need to dl media center 2005, but i really need to find out how to get the product key if i can.

If you need any additional information please tell me.