havenīt tried it, probably have to buy it to get the full number. This one will apparently tell you about everything on your system.
Also found this recommendation:
"You can try 'System Info' available at
www.gtopala.com. It is a small tool, just 1.26 MB, even needn't installation process. It gives all HW/SW information of windows PC. This program has windows explorer like view with two panes. In left pane expand 'Applications'. Select 'Microsoft Office'. In right pan you can get the Product Key along with other informations."