Just this morning I did a suggested security update on my 1.83 Macbook running 10.4.7. A restart was required....after the restart I got a white screen with a series of colored vertical lines on the right side. I did another restart and got a normal startup chime and the computer hung at a blue screen. That's it. Several restarts attempted, PMU reset, attempted to boot from install disk - all no joy. Blue screen on every attempt. This machine had the logic board replaced 1 month ago so I called Applecare. They suggest that the "recent" logic board repair was not done correctly. I call BS on that but am now stuck with taking the machine for repair. Called my local Apple guy and he says that I am the second customer to have had "THE EXACT SAME THING HAPPEN AFTER DOING A SECURITY UPDATE." My advice is to hold off on doing this update.