Hello djuser,
It is recommended that you reinstall using Genuine Office install media. The thinking behind that recommedation is that if the key was pirated, it is possible that the Pirates may have added Malware to the software.
However, it is possible to change the Product Key, without reinstalling Office, if that is what you wish. Please see the support article "
How to change the product key for Office XP, for Office 2003, and for the 2007 Office system"
http://support.microsoft.com/kb/895456 for details.
Thank you,
Darin MS