I do not believe there was a provison in XP Pro to go back to the XP home. So you will most likely need to backup what ya need, and reinstall from scratch. Going from Home to Pro does not follow the upgrade path that going from 95 -ME would. They are not the same systems. Similar, but not.
But why go back to the Home version anyway if you got Xp Pro? the Pro version should be more stable than the Home version. the better parts of Pro is the SMP abilities, which i am sure your not taking advantage of. Most people with Pro dont use it for that anyway.
backup all your files to CD. used IE to export your bookmarks,cookies.
and just install Home from scratch