Niche feature in Microsoft Office for mac is it not allowed a serial number (for personal license) installed in more than 1 computer in the same network. If that already happen you need to change your serial number, here is some tricks to do that :

1. Quit all open Microsoft Office applications
2. Go to directory /Applications/Microsoft Office 2008/Office/
3. rm OfficePID.plist
   
4. Go to directory ~/Library/Preferences/Microsoft/Office 2008/
5. rm Microsoft Office 2008 Settings.plist
   
6. Launch one of the Office applications to go through the Setup Assistant and enter the new license key