Typically, Microsoft Office suites can be installed on two machines---one for your desktop and one for a laptop. This allows you to share Office between two computers, as long as they are used by one person. You must enter the Product Key to activate each version. However, if you own a Home and Student or Military Appreciation edition, you can share your copy with up to three computers in a household. Microsoft also allows you to transfer Office to a new computer, provided you uninstall it from your old computer.
Uninstall Microsoft Office on your old computer. You can do this by going to the "Control Panel" and clicking "Uninstall a Program" or "Add/Remove Programs." Select "Microsoft Office" and click "Install" or "Remove."
Install your Office software on your replacement computer. This will work as long as the number of computers your copy of Office is installed on meets the license terms (no more than two for a regular suite).
Enter the 25-digit Product Key. Your Product Key will be on the sticker of the CD case, packaging or the Certificate of Authenticity if you bought it from a retail store. If you bought Office online, look for the Product Key in the confirmation e-mail.
Call Microsoft for confirmation if prompted to do so. You may need to explain your situation (e.g., you bought a new computer) and Microsoft will provide a new activation code.