I recently installed Office 2007 on my laptop with the wrong product key and found instructions on Microsofts knowledge base for resetting the key but they didn't work. After 15 minutes of faffing around it eventually occurred to me that the registry setting for accomplishing this might be in a different place on an Vista x64 and sure enough it was.

 So, if you need to change the office 2007 product key and are using Vista x64 then try the following:-

• Located the registry key HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\Office\12.0\Registration
• Delete the DigitalProductID and ProductID subkeys
• Exit the registry editor and try an Office App again

x32 UPDATE:
Quite a few people have contacted me for information on changing the product key for Office XP, Office 2003 and Office 2007 on 32 bit systems in particular Windows XP. I think this is quite well covered in other articles on the Internet (see http://support.microsoft.com/kb/895456) so I don't plan on repeating the information. BUT if you do find something that isn't covered by the Knowledgebase article and your aren't requesting a free key then let me know and I will see whether I can help.