Cruiseman is correct BadgerBoy13, you should go to the Windows XP Newsgroup for this. .....
But to answer your question, you can't, unless you give your old computer to your dad. Your licensed copy of Windows XP is directly tied to your old computer's manufacturer, motherboard chipset and Windows XP Product Key, and therefore cannot be installed on another computer for more than 15 days. In other words, you won't be able to activate your old copy of Windows XP on your dad's old computer.
Hope that helps.
Elliot