When you install a service pack, Window XP removes the ability to use the Start Windows Using Last Known Good Configuration to prevent you from using it to uninstall the service pack and create severe problems for your installation.
To uninstall a Windows XP service pack:
1. Control Panel / Add or Remove Programs.
2. Select Windows XP Service Pack n from the list of installed programs and press Remove.
NOTE: If you did NOT select the option to archive files during the service pack upgrade, you can NOT remove it.
NOTE: If you subsequently installed updates to Internet Explorer, the Windows XP service pack entry may not be listed. To restore the listing, remove the Internet Explorer updates first.