I installed my copy of Office 2008 on my mac, but unlike the Home and Student edition I bought, the Special edition only allows one main installation. I didn't know that, since I got that version through a special offer from Microsoft. I assumed it too would have 3 installation limit. Since I already installed it on my two Macs, I needed to uninstall one and install a new copy. But I found another user (Link) who had a similar issue and his solution was simpler and less time consuming. I only need to go through the setup assistant again to enter my new key.
To change the serial number, product key, CD key, license key, or whatever you want to call the string of characters required to activate a Microsoft product on an existing installation of Office 2008 for Mac, follow these simple steps:
1. Quit all open Office applications to prevent them from clobbering your work
2. Delete the file /Applications/Microsoft Office 2008/Office/OfficePID.plist
3. Delete the file ~/Library/Preferences/Microsoft/Office 2008/Microsoft Office 2008 Settings.plist
4. Launch one of the Office applications to go through the Setup Assistant and enter the new license key