Product id vs. Product Key

Last month I bought three copies of Office 2007 as I have three systems at my office. I already installed two of them and just trying to install the third one. But one thing went wrong. I forgot to note which two are already installed from the CD case. I thought i could tell by accessing the Product ID, but the ID is not the same as the KEY. Now I wanted to know whether there is any way I can get to know which two of them are installed and which one is left?

Help appreciated.

To know which two of them are installed you will need to know first their product keys which is stored in encrypted registry key. Now in order to get those keys, you will need install a third party software named Belarc Advisor on both of those computers.