MS Productk Key change

I recently lost my hard drive and had to re-install my software. I had misplaced my Office 2003 CD and a friend let me borrow one he thought had a good product key. Unforunately, when I attempted to register, the maximum number of users had been met.

I have now located my MSOFFICE CD.

My question is, do I need to reinstall or can I change the product key and register again?

Thanks.