The COA on the system is for Windows, not for Office.  The Product Key for Office is on the envelope or case with the reinstall CD for Office.
I just recently purchased a Dell Studio. Microsoft 7 Home Premium was preinstalled. I have a reinstallation DVD. When I open Microsoft Office, it asks me for the  25-Character Product Key . I enter the key that is on the COA on my computer but it states it is invalid. I searched the system through Belarc and a different product key is showing through their advisor. This code does not work either.  Since the product was preinstalled at Dell, do I need the  25-Character Product Key. If not, then how do I keep the system from asking me for this everytime I open Word.  Thanks - Karen 
 25-Character Product Key 