Back at home now. Since I had a number of other utilities installed, I decided to try to see if this was just a third-party conflict. So after reinstalling the system from scratch, I started with iLeopard, intending to install my other utilities one at a time and test things out. The system immediately ground to a halt again. So now I'm back up and running after two clean system restores. This might work fine for you, but I wish I had heeded the warnings and not taken the risk. More detail on the troubleshooting tab.

Further detail for those who are interested: It was immediately apparent that something was wrong, because as applications crashed, the buttons of the dialogue boxes no longer displayed properly. Instead there was a missing-pic image, similar to what happens when a pic doesn't load properly in some web browsers. The image was a simple vertical gradient, going from a Facebook shade of blue at the top to pastel cyan at the bottom, with the sans-serif text "CUI" in orange at the top and a large question mark in red at the bottom. I don't know whether this placeholder image came from iLeopard or the system. Again, to repeat, all applications, including System Preferences, would crash immediately upon loading; the Finder crashed immediately upon attempting to open a new window. I've been using and customizing Macs since System 6, and have never seen a system this completely wrecked, this quickly. I am using an older system: 10.5.8 on a dual G4, which according to the information given should be supported. I'm very disappointed that a developer would release something this unstable, even as a beta, and allow it to just sit there on people's servers for a year, unfixed, just waiting to cause problems.