I received the error, "This Service Pack requires the machine to be on AC Power before setup starts…" when I install Service Pack 2 for Windows XP.
The Setup program for Windows XP Service Pack 2 (SP2) requires that your computer uses AC power. If the battery power runs out during installation, the update cannot be completed. If this occurs, you might not be able to restore the operating system to its previous state.
Solution: To resolve this issue, connect your computer to an AC power source, such as an electrical outlet, and then run Setup.
For more information about this issue, see Article 883609, "This Service Pack requires the machine to be on AC Power before setup starts," at the Microsoft Knowledge Base (http://go.microsoft.com/fwlink/?LinkId=70662).