99% of the problems I've come across with Leopard involve updating an older system, especially 10.3 and below. I once had a similar problem with 10.2, in which after updating, the login screen showed it as being 10.2 as well as the About This Mac info, however, it had Time Machine/BootCamp and everything else related to Leopard installed on it. Actually, that instance was even from a Disk Utility clone, erase, fresh install, restore method which produced the weirdest results I had ever seen.
Pretty much your best bet is to start backing up any personal data you have to an external device, run the installer again, but chose "Erase and Install" just before the install takes place through the "Options" button to perform a clean, fresh install of the OS thus removing the older sys files that conflict with Leopard.