i had the same problem - sometimes 3,2 or 1 minutes left.
then one time i just tried to reboot after that "unfinished install" and - big surprise - leopard was running.

i asked myself if everything could be ok with that install - but i guess only the system optimization was missing - so i ran that manually with onyx.

now i have a few issues - but they seem to be usual issues for a leopard on pc install (usb-storage problems, soundproblems) or at least usual issues for leopard in general (safari crashes, time machine doesnt work).

so mabe you just try to reboot after such a unfinished install and look what happens then...