Uncle Bill would prefer that you have new and individual copies of Windows XP for each of your computers. MoJo feels that his three children will not starve or suffer shopping at thrift shops if you decided to use a gift Santa brought you on more than one computer.
Maybe he only has 2 kids. Well, who cares! Uncle Bill's kids will be just fine.
Each copy of WinXP has to be activated at the MS site so that it can't be installed on more than one computer at the same time unless one is willing to do something illegal. If you do not activate after 30 days it quits working.