Uninstall old SEP client using cleanwipe utility.
Uninstall Windows Installer.
Delete Folders from C:\Program Files\Symantec
Delete Folders from C:\Program Files\Common Files\Symantec Shared
Delete Folders from C:\Documents and Settings\All Users\Application Data\Symantec
Try to install Unmanaged SEP & replace the Sylink with the Managed.
This works.
Regards...
Ramji Iyyer