Ok have a look at the link i posted (Start at "What if the product key does not work or you were not able to find the genuine product key?") and see if any of those options are any good to you.
If the COA sticker is damaged, in that case you need to use the recovery partition to reinstall Windows 7. It will take your PC to the factory state, wiping all your data and software installed by you. This will exempt the need to use the product key to activate your Windows 7 installation.
Make sure you backup all your data before trying this.
If your recovery partition is not available or damaged. In that case contact the PC manufacturer and request for a recovery disc set that you can use to reinstall Windows 7. There might be a small charge for shipping and handling. This will also exempt the need to use the product key.
If these dont work you may have no choice but to purchase a new key,a bit of research on the net doesnt show up any other solutions i can find..hope one of the above works for you.