In my previous post on this topic, I mentioned that Windows XP Service Pack 2 will copy the Administrators settings to Default User (see sections B & C). The new profile copy is the default behavior in SP2 with no built-in way to disable it. To disable it you had to request the hotfix from KB887816 from Microsoft Customer Support.
This has changed with Windows XP Service Pack 3. The fix from KB887816 is included in SP3. (See KB946480 for a list of fixes included in SP3.) Therefore, the behavior for SP3 is no profile copy by default. To enable the profile copy, you now have to add UpdateServerProfileDirectory=1 to the [Unattended] section of Sysprep.inf before running Sysprep. Unfortunately, due to an oversight you will not find this documented in the Microsoft Windows Preinstallation Reference (ref.chm) included with the SP3 Deployment Tools. KB887816 is currently one of the only references for this setting, as far as I know. (KB908784 also references this setting with respect to a problem that the profile copy can cause on Windows Server 2003)