First of all you need a working installation of Mac OS X 10.5.7.
For the first steps do not use your working installation from your EFI-Mac. Something could go wrong and you could loose all your data.
Make a clean and fresh installation from scratch on a separate hardisk. I made it this way. You can also try a new installation a timemachine backup (Mac OS X DVD Installer -> Menu).
Download the following stuff:
Find a USB-Stick. 128MB should be enough. Mine has 512MB. It's a Kingston DataTraveler.