Assuming that your dad has a retail version, here’s what you need to do:
Open Word or Excel. . .Click the Microsoft Office Button , and then click Word or Excel Options. .. .Click Resources, and then click Activate.
Follow the instructions in the Activation Wizard.
You can telephone an Activation Center and activate your product with the help of a customer service representative.
In the Activation Wizard, select the country/region where you reside and where you plan to use the product.
The wizard provides one or more telephone numbers appropriate for your selection. Use the number provided on your Activation Wizard screen to call the Microsoft Activation Center.
Call the Activation Center.
The customer service representative asks you for your Installation ID (displayed on your screen) and other relevant information. (Installation ID is different from the Product Key) After your installation ID is verified, you receive a confirmation ID.
In the Activation Wizard, type the confirmation ID in the spaces provided at the bottom of the screen, and then press ENTER.
That’s all.