All of the HP PSC-class multifunction devices of that age have problems with Leopard which are known, and which will not be fixed because they have been abandoned by HP. The best that you are likely to see is to get print-only drivers; scanner drivers will
not be updated. Some are supported by 3rd-party scanner systems, but some others, as you have discovered, are not.
The solution is simple: buy another device or, where possible, drop back to Tiger. You are extremely unlikely to get this one to work with Leopard.
I farmed my old PSC-class MFD out to a WinXP machine. (It didn't work properly with Vista, either.)