If you get a blue screen of death after using the Software Updater in Leopard, this can usually be fixed by rebooting in "safe mode". You boot in safe mode by holding down the Shift key when you Mac is booting. Once you get the blue screen, reboot by holding down the power button for several seconds. Once the boot starts, let go of the power and hold down the Shift key on the keyboard.

Once you have rebooted in safe mode. Boot again as you would regularly. You should now be able to boot without the blue screen of death. I know it sounds crazy, but Apple's listing this on their own site: http://support.apple.com/kb/HT1564.