Não sei quanto disso se traduz no Windows XP, mas sinto que devo mencioná-lo. No meu entender, esse local só pode ser alterado durante a instalação .
Tanto quanto eu entendo, ele também é strongmente desencorajado .
This setting should be used only in a test environment. By changing the default location of the user profile directories or program data folders to a volume other than the System volume, you will not be able to service your Windows installation. Any updates, fixes, or service packs will fail to be applied to the installation. Microsoft does not recommend that you change the location of the user profile directories or program data folders.