To unregister it off your laptop, just remove Office from the laptop.
Be sure to run the Remove Office tool in the apps/ms office folder.
Then use the same registration number on the Mac Pro (I hope you tracked
which one you installed where).
A temporary measure would be to disconnect the laptop from your local
network while installing on the Mac Pro, and then you should be able to
use the duplicate registration number, if you want to get the Mac Pro
set up before binning the laptop. But you will never be able to run
both versions at the same time--and that means, no Excel on the laptop
while using Word on the MacPro, etc.
If you didn't track which number you used on which computer--there is no
way to tell from the Product ID in the About... window, *but*, the same
registration number will create a Product ID that is almost the same.
So if you have two Product IDs that are the same except for the last 5
digits, then you used the same registration number on both those computers.
If you already installed on the MacPro and need to switch keys to match
the one on the laptop:
"At least for later versions of Office 2004, launching the Remove Office
application, then holding down the Option key at the first dialog will
change the Continue Button to read Remove Licensing Information Only.
Click that button and you'll be able to enter your new key."
http://www.mcgimpsey.com/macoffice/office/pid.html