Don't know if it will really works, but when I installed the SP1 service pack (previous win xp regnumber change) it ask you to choose betwen 2 options:
1.- saving the old files to desinstall the sp1 if you like, but it will require more hd space
2.- installing without uninstall option
But, if the unisnstall option will be good for anything, really don't know.