you f'ed up the registry keys. It was still showing in the Add/Remove Programs because it wasnt removed properly. So windows still thinks it is in Location A. Now you installed it again to Location B. So when windows tries to access it looks at Location A and goes "where the f did it go?" searches and finds it at Location B links it and it is a whole big 'ol jumble of mess.
Easy way: Reinstall XP start Fresh.
Hard way: undo what you undid and fix all the registry entries.